I start playing songs to my son James from the moment he is born. If I’m given the job of rocking him back to sleep, I usually put on reggae: Junior Murvin’s Police and Thieves and Dawn Penn’s You Don’t Love Me (No, No, No) tend to do the trick. If my partner Ginny or I sing along to whatever is on the CD player, it brings him a gurgling kind of delight. In this, he is – obviously – no different from any other child. But not long after his first birthday, I get a sharp sense that music might speak to him in a particularly vivid, mood-altering way.

I play James the title track of Clear Spot by Captain Beefheart and the Magic Band only once. Its mixture of discordant guitar, growling vocals and knock-kneed drumming, I suspect, might strike him as curious and funny, like a big, monster-centred production number from Sesame Street or The Muppet Show. But it has pretty much the reverse effect: within a few seconds, his face is suddenly filled with an expression of absolute panic, he screams in protest, and I instantly know I have to turn it off and never put it on again.

When James is just over 18 months old, he starts to walk, and he begins to talk a few months afterwards – both a little later than our parenting books suggest. There are very quirky aspects of the way he interacts with the world. When he is happy or excited, he flaps his arms – sometimes so animatedly that the effort required completely consumes him. Everyone notices this. Our friends think it’s cute; so, therefore, do we.

On Sundays, I usually take him to an expanse of pebbles and rocks on a river bend about 15 minutes’ walk from where we live. He walks some of the way; when he gets tired, I carry him on my shoulders. Once there, he happily plonks stones into the water. Doing so never seems to become boring or reach any kind of conclusion. And whether we stay there for 30 minutes or two hours, the suggestion that it is time to leave is always met with a burst of rage and upset, so loud and traumatic that it sometimes takes both of us the rest of the day to recover.

Some of his more benign rituals are about music, or the stuff around it. One happens at the start of every day. “Fi pee bus,” he says. He then waddles over to the bookcase, where there are around 70 books about the Beatles. (It’s that kind of house – I am an obsessive fan who spent the first 15 or so years of my working life as a music writer; Ginny used to be a press officer at Parlophone, the Beatles’ first record label, and we first met at a Beatles launch). He tries to pick up one big book that breaks down the band’s career into a daily chronicle, and then gets very excited when I intervene, stopping at a page that contains a picture of the coach that had been driven around the West Country during the making of Magical Mystery Tour. This carries on for weeks, and I realise that he is asking to “find a picture of the bus”.

When James is about to turn two, we buy him a tiny toy piano. Knowing that as long as he plays the white keys it will sound in tune with anything in the key of C, I show him how to play random notes in a 4/4 rhythm. As I play the guitar, for five or six minutes at a time, he hammers the keys in perfect time. Every eight or so bars, he looks up with an expression of complete delight.

One day, I put on the Beatles’ Paperback Writer. He fixes me with an intense and very happy look and does his best to precisely repeat all the words, which he has obviously learned phonetically, soaking them up whenever he hears them. He does the entire song, pretty much faultlessly: “Dee Sir Madah, will you ree mah book …”

This seems a thing of wonder. By now, though, I am also experiencing pangs of worry. By the standards of the other kids at his nursery, he is quiet and withdrawn. A few times, people we meet say he seems “placid”, a word that always rattles around my head for days. Sometimes, I hear other kids of his age talking, and suddenly think, James doesn’t do that. But we are first-time parents: between the fact that he talks and walks and those few apparently striking abilities, we have enough to bat away any worries and blithely muddle on.

Just before James turns three, he is joined by a sister, Rosa. When the day comes to bring Ginny and Rosa home from the hospital, James picks his way to where they are with an air of hesitation. When Ginny extends her arms so Rosa is closer to him, he looks at her with a curiosity that seems to be combined with shock, and pulls back. Back at home a couple of weeks later, he bursts into tears. What he says leaves us in no doubt about what the matter is: “Why Rosa?”

Two months pass, and we move house, from Hay-on-Wye, in Wales, to Frome, in Somerset – another town seemingly filled with resting actors, bohemians and members of the Green party, only bigger. James is obviously thrown by the move, and the arrival of his sister: he is quiet – often to the point of silence – and sometimes unsettlingly distant. I notice that even the things that used to be certain to lift his mood – music, a walk to the park, the right programme on CBeebies – no longer seem to work.

In the early summer, we spend a week in the Welsh seaside resort of Tenby. One day, we go to Pembroke Castle. I take James to the top of one of the towers, hold him up and ask: “What can you see. James? Look how small the cars are. Can you see the houses?” He doesn’t follow any of this, at all. His eyes are glazed, and in a faint whisper, he comes out with a burbling stream of words he has seemingly soaked up from the TV, like a human tape recorder: “When the night is black and the stars are bright / And the sea is dark and deep … Time for Teletubbies / Time for Teletubbies / Time for Teletubbies.”

That night, we put him to bed, and I go back to check on him half an hour later. As I climb the stairs, I hear the same murmured scripts, and feel a mounting sense of panic. Inside his room, I find him lying faceup with his eyes focused nowhere in particular. It is as if he is in a trance. And then it all comes again: “When the night is black and the stars are bright / And the sea is dark and deep …” I put my face in front of his and say his name, but nothing changes. In some very unsettling, inexplicable way, he is not here.

I try to make light of all this, but I sound like a facetious idiot. “James has gone bonkers,” I say. We have no idea whether we should be concerned. But amid nappies, wine, baby food and broken sleep, we blithely muddle on.

Then, back in Frome, something happens. One afternoon, Ginny returns from the nursery James has been going to for three weeks. At drop-off and pickup, we have started to detect a feeling of unease from the people we speak to.

“They’ve just told me a load of things about him,” she says. She looks out of the window; I sense that if she turns her eyes to me, she’ll lose it.

That is exactly what happens: everything else comes out in panicked fragments. When they call the kids to snack time or tell them they can go outside, he doesn’t notice. When the others all take off their shoes, or get ready to leave, he never does anything like that. They say that the TV talk is just about all he comes out with.

By now, she is crying. I get up from my chair and hold her. “They think he might be autistic,” she says. Everything seems to stop.

* * *

What happens next is brutal. My perception of James changes. Before, he was simply full of life and joy and quirks. Now, he suddenly looks lost and fragile. Because most of what we are told by experts and professionals is about supposedly normal things that he is unable to do, everything he says or does is freighted with worrying portents.

There were plenty of signs of James’s autism, though we either had not seen them, or failed to appreciate their significance. Now, as we pick through websites and pamphlets, there is a terrifying clarity to everything.

“Autistic people have difficulties with interpreting both verbal and non-verbal language like gestures or tone of voice. Some autistic people are unable to speak or have limited speech.”

We should have noticed that James neither points, nor waves hello or goodbye. If we try to draw his visual attention to something, getting him to focus on the same thing is often impossible. In general, imitation is a strange, alien skill.

At first, when we asked him questions, he didn’t answer; now, he has taken to responding to them with a strange truncated echo:

“Do you want a biscuit or some cake?”

“Biscuit or some cake?”

“After lunch, do you want to go into town?”

“Go into town?”

“Autistic people may also repeat movements such as hand flapping, rocking or the repetitive use of an object such as twirling a pen or opening and closing a door.”

The way he flaps his arms when he is excited or happy – one of his most endearing traits – is “self-stimulatory behaviour” or “stimming”, which is how some autistic people use movement to soothe themselves.

On and on it goes: a great list of problems that strips any magic away and leaves everything pathologised. His habit of precisely reciting whole passages from books or TV programmes is explained in one of the books I buy. “The repetition of words may have little meaning for the child and this empty echoing, like a parrot, is called ‘echolalia’,” it says.

And there is one realisation that really stings. Change can be hugely difficult for autistic people, and there is a whole world of online advice about “transition strategies”. We have just presented James with two classic examples: the arrival of his sister, and a house move. No wonder he has been tired and withdrawn.

It is September, a time of the year I always associate with fun receding and the demands of the real world suddenly tightening their grip. After going to see our GP, an endless series of form-filling for us and developmental tests for James begins. An educational psychologist comes to the house with a box of paraphernalia and tries – in vain, mostly – to get James to build towers out of small blocks and pick up tiny beads, while she puts ticks or crosses on her paperwork. There are two meetings with speech and language specialists. Eventually, Ginny and I sit for a long scripted interview with an NHS paediatrician, which results in a completely obvious verdict that arrives in the post.

“In conclusion,” it says, “James meets the criterion for an autistic spectrum disorder.” Implicitly, the brisk two-page letter has another message: that all the fuss and questioning is now pretty much over, and we are going to have to cope as best we can.

Life feels panicked, myopic and weird: all about James, and what we might be able to do to help him. If I pick a fear, Google will instantly deliver. Only 22% of autistic people, I read, have a job; 80% of the parents of autistic children say they are often “stretched beyond their limits”. The parents of children with an autism diagnosis have a much greater chance of divorce than couples as a whole.

What keeps us going is the delight and fascination James gets from music. Again and again, he takes us back to the Beatles. James sings along to I Am the Walrus without a single mistake: he has perfectly memorised all of its details. When he puts on Paperback Writer, he knows all the words and the falsetto parts that come after each verse. Can’t Buy Me Love and All My Loving make him perform joyous circuits of the kitchen. Because he can confidently operate the iPod, he habitually flits through all the Beatles’ albums, and finds the other songs that are becoming part of his personal repertoire: Help!, Here Comes The Sun, I Feel Fine.

One Saturday, I buy him the DVD of the Yellow Submarine movie: that 90-minute fantasia in which the evil Blue Meanies invade a harmonious utopia called Pepperland, outlaw music and turn everyone to stone. Then, summoned from Liverpool by an escapee called Old Fred, John, Paul, George and Ringo set out on an odyssey that leads to Pepperland’s liberation and the Meanies’ conversion to love and peace.

I know when something he watches is a hit: he puts his face no more than six inches from whichever screen he is looking at, and it is impossible to pull him away. He adores the When I’m Sixty-Four sequence, when the Beatles shrink back to being little kids and then rapidly age, sprouting huge grey beards and moustaches as the animated clocks speed forwards. I find it on YouTube: for at least a week, at least 10 times a day, he responds to it with a beaming grin and delighted hand- flapping.

In time, he will meticulously memorise the year each of their albums was made, and where each sits in the sequence, so that Please Please Me is number one, and Let It Be is album 12. Since I was James’s age, I have had a deep sense of the Beatles’ brilliance; now he has discovered it, too.

* * *

In the months leading up to James’s fifth birthday, we are plunged into a grinding struggle with our local council over what he’ll require if he’s going to go to a mainstream school. He needs a whole system of one-to-one support, therapy for his sensory and language issues, and much more besides. Our request for a formal assessment is turned down flat, so we have to launch a legal appeal. Then, just as we are about to reach a tribunal, we are told that the council will give us roughly what we want. A long run of arduous meetings then starts; when I get back from the last one, I burst into tears.

Finally, a huge day arrives: James in his Clarks shoes, white polo shirt, bottle-green jumper and grey trousers. He has had a handful of practice visits to his new school, but that doesn’t ease the anxiety we’re all feeling. The first time I drop him off, all I can think of is a Tears for Fears lyric that I used to think of as being unintentionally funny, but which now seems as serious as its writer intended: “Went to school and I was very nervous / No one knew me / No one knew me”. In the classroom, he does what he always does when he is overwhelmed, alternating between hand-flapping and whispered echolalia, and small stretches of stillness and silence. Straight away, I notice the things that really highlight James’s difference: the instinctive way that most of the kids effortlessly settle into groups; the fact that clapped hands or a single word – “Right!” – instantly get their attention.

Because he can’t tell me much about what he does at school, I look for other signs of what he is getting out of it, and how it makes him feel. There is not much to go on – but one morning, when I ask him who else in his class, he slowly lists 10 or 11 names. This leaves Ginny and me in bits.

These are the kinds of things that keep us going: for the moment, life is not about huge leaps or hideous crises, but small steps forward, and a fragile calm.

In time, though, our fear and worry come roaring back. James’s system of support starts to slip, and so does his mood. There are suggestions that he should have his own daily schedule with little or no academic elements in it, and hints that he should go to a special school. He resurfaces thanks to a change of personnel and a brilliant idea: rewarding him for doing short bursts of work with five or 10 minutes in the hall, where the speaker system blares out I Am the Walrus, Yellow Submarine and the Clash’s London Calling.

Before long, another big change comes into view. In Somerset, we have middle schools, which amount to the terrifying prospect of nine-year-olds suddenly having to cope with timetables and regular changes of classroom. For James, some of this will be smoothed over by his tutors, but it will be another rupture for him – and, for Ginny and me, an anxiety about whether everything will work out that slowly builds through the summer.

For the last two years, I have been chairing political discussions at Glastonbury, in the big top known as the Left Field. Most of what happens there is put together by the singer Billy Bragg. He lives on the Dorset coast; when the time comes to discuss who we fancy asking to take part and what they ought to discuss, he graciously drives up to Frome, and spends several hours at our house, thrashing everything out.

The 14-year-old who bought his first two albums is still inside me somewhere: when I offer him tea and custard creams and we throw around the names of climate protesters, Labour MPs and trade union leaders, I feel a frisson of starstruck amazement. Back then, I used to listen to his songs while I did my morning paper round. Now, here he is, in the corner of the kitchen where I keep my acoustic guitar.

He asks Ginny and me how James is doing. We mention that as we have started to talk to him about the end of his time at his first school and what will happen at the next one, he has been singing a defiant three-word mantra. When we explain this, Billy reaches for the guitar and combines James’s line with two chords; the result sounds like the chorus of an as-yet-unwritten parody of the Clash.

I go and get the microphone and amplifier we keep in the music room. Sitting next to him, James tentatively joins in, as we film it. “Louder!” says Billy.

On a Thursday morning two weeks later, his class is honoured with a leavers’ assembly. Behind-the-scenes lobbying by James’s new tutor Lisa has made sure that it has the perfect opening. Ginny and I watch from the back of the school hall. A big projection screen is unfurled, the lights are dimmed, and there is James’s 19-second punk-rock answer to all his ups and downs.

He sings it with a glint of mischief:

No more school, no more school
No more school, no more school
No more school, no more school
No
More
School

* * *

For a long time, a thought has been nagging at me: if James can read so prodigiously, and has such an ear for music, isn’t it time we found someone to teach him an instrument? Now, with the return of some stability and calm, we decide to do something.

I take a Monday morning off work, and start searching online for “special needs piano teachers”. I can find only one, and she lives an hour away. The fact that this will entail a two-hour round trip doesn’t even occur to me: my mind is in super-drive, and it feels like something good is going to happen.

Hajnalka, who is Hungarian, lives in two rooms. At right angles to a small kitchenette, next to shelves full of sheet music, there is a Yamaha keyboard fitted with a music stand, and two chairs. I sit near the front door. James takes his place at the keyboard. “What can you play, James?” she says. I was expecting this, so we have prepared something: Twinkle, Twinkle, Little Star in C, which took him about 90 seconds to learn.

He plays it. “OK,” she says. “Let me try something.”

The keyboard has its own drum machine. She sets it to a fast tempo – boom-crash/boom-boom-crash – and asks James to play again.

He does it beat perfect.

She then changes the rhythm setting to a waltz – boom-crash-crash/boom-crash-crash – and asks James to do it again. Putting music in a different time signature is not easy: all the emphases change. But he does it as a matter of instinct.

“Amazing,” she says.

“Amazing,” I reply, and I really am amazed.

Getting James to practise at home, though, is hard. Most of the pieces – Girls and Boys, Come Out to Play, Austrian waltz, Oats and Beans – are rather charmless. He sometimes reacts with shrieks: “I DON’T WANT TO DO KEYBOARD PRACTICE I WANT TO GO ON THE COMPUTER.” To maintain his cooperation, I soon have to use one of the token boards he uses at school. The best is based on the sleeve art for Abbey Road. Each Beatle has a little Velcro dot on his back, and as James gets each part of a task right, they are stuck to the zebra crossing – first George, then Paul, then Ringo, then John, at which point he earns a break.

Not long after the lessons have started, I teach him something. First on the iPod, and then Spotify, James has become fixated on yet another song: I’m Waiting for the Man by the Velvet Underground, Lou Reed’s insouciant portrait of scoring heroin in uptown Manhattan.

I think I know why he likes it: it is something to do with its amazingly repetitive music. This was a Velvet speciality. In a different form, it reached its peak on Sister Ray, the 17-minute closing track on their second album, before surfacing spectacularly in the mesmerising version of What Goes On included on 1969: The Velvet Underground Live.

Music with this incessant quality tends to impress James – he has a similar obsession with the Kraftwerk song Autobahn. It seems to reflect the autistic fondness for what the diagnostic manuals call “rigid and repetitive” behaviour, and his habit of playing small parts of songs over and over. Maybe there is something synaesthesia-ish at work, to do with some autistic people’s liking for enclosed spaces: the sense that in creating such a thrillingly confined feeling, repetitive songs deliver a deep comfort.

Or maybe, like millions of other people, he just likes songs like that. Anyway, I know how to play I’m Waiting for the Man. I set the keyboard’s drum machine to “Rock 2” – boom-crash/boom-boom-crash – and off we go. Five chords, and five verses: while I chug along on the guitar, James only has to trigger the correct music with his left hand – E, A, G sharp, F sharp – and shout and sing the words, which he loves: even as phonetic sounds, he clearly thinks such lines as “PR shoes and a big straw hat” are fantastic. If we were in a recording studio, I would say that he nails it on the first take.

* * *

Another September. James starts at middle school, in a new maroon uniform. As a result, things still feel fragile and fitful, but James starts to make progress. His weekly highlight is the music class led by a teacher called Miss Parsons, who looks like a retired ballet dancer, and is full of warmth, but also unbelievably ordered and together. Miss Parsons always shows James songs and riffs he can play on the keyboard, and, at the end of each lesson, she also lets him select songs to play from Spotify to the whole class. He is regularly allowed to spend some of his lunch break doing as he pleases with the instruments in the music room.

Eventually, Miss Parsons tells us about her department’s annual production. It’s called Oakfield’s Got Talent, and she wonders whether James might perform? When I ask him, I get a fervent yes; to reduce the chances of anything unexpected happening, she agrees to the suggestion that I should accompany him on an acoustic guitar.

It happens just over a fortnight later. We decide to perform When the Saints Go Marching In and I’m Waiting for the Man, both of which have been extensively practised, and then played in three dress rehearsals.

We will be the first act on. The school hall is packed, and I am full of churning anxiety. James does not seem nervous at all.

An introduction – “This is James Harris, and his dad!” – and off we go. The keyboard’s swing jazz preset begins gurgling and thumping away and James plays the first notes of The Saints on its trumpet preset, whereupon the mums and dads in the audience instantly start clapping along, on the “on” beat. This throws him. He drops out of the rhythm, then drops back in, then drops out again. We may be about to implode.

But, as ever, James is great at correcting his mistakes. At the start of our second and final verse, everything coheres, and he plays the rest perfectly. A beatific smile breaks out on his face. He presses the “end” button on the keyboard, and in a final flurry of booms and crashes and one last chord, we finish. The applause this triggers is quite something: a great warm burst of encouragement and appreciation that makes me well up.

I reach for a piece of paper that is serving as a cue card, and James reads it out: “This next song was originally by the Velvet Underground, and it’s called” – he then slows down – “I’m. Waiting. For. The. Man.”

When we play it, James sounds like Mark E Smith from the Fall, barking out the words, and rising to the conclusion of each verse – “Oh, I’m waiting for mah man” – with a loud sense of triumph. A few times, he drifts away from the microphone, and yells the words into the air. We have worked out a procedure for this: I say “Microphone! Microphone!” out of the side of my mouth, and he returns to the right spot.

I don’t know if many of the audience quite understand what they are listening to: a less-than-wholesome song about copping dope in 1960s Manhattan, the grimness of withdrawal, and the rapturous pleasure of yet another hit of heroin. But they like it: we get a second round of applause, and I do that showbiz thing of camply extending my arm in James’s direction. There are a few whoops, and he picks his way down the wooden stairs to the right of us, before taking a seat in the audience.

Ginny and Rosa are there. To us, the meaning of the six minutes James and I have just spent on the stage is pretty obvious. If you are repeatedly told what your child can’t do, it starts to eat at you. Certain words hover over you: “severe”, “profound”, “impairment”. You miss superlatives; whatever successes your child achieves, they don’t tend to feel like the same ones other kids experience. But here is something James can do – brilliantly, fantastically, wonderfully – on the same terms as everyone else. Better still, he loves doing it, and it makes him the centre of attention.

It is a gorgeous summer evening, and everything feels as if it is surrounded by a lovely glow. When we get home, James does not sleep, but I do not mind at all. “I want to do that again,” he says. “I want to do that again!”

• This is an edited extract from Maybe I’m Amazed: A Story of Love and Connection in 10 Songs by John Harris (John Murray Press, £16.99). To support the Guardian and Observer, order your copy from guardianbookshop.com. Delivery charges may apply.