Thanks for your replies. I will look out for the Michael Aaron book next time I'm in the music shop.
Good point, Jo.clarinet. I do play both recorder and piano (and clarinet and guitar and sing...although none of these to a teaching standard). On recorder, he refuses to be shown how to do things initially, but is open to ad hoc suggestions such as, "Make sure you're covering the holes properly" or "try turning your hand a little." Thanks to the confidence he got learning at home he is now having lessons on recorder at school, which takes some of the pressure off.
Thankfully he is fairly happy to be guided on piano. He positively refuses to have formal piano lessons, but I hope if he enjoys the self-teaching he will change his mind once he has some confidence - or if not, nothing lost. Although I am aware there are pitfalls to self teaching, given a choice between no knowledge of music and some learned haphazardly, I would prefer the latter for him.
If it sounds as though I'm afraid to put him off, it's true! He used to despise music, and even as a baby cried if there was any music playing. I am so astonished and delighted that he has taken an interest at all that I am happy to let him discover it on his own (having tried unsuccessfully to foster a love of music in him for the last 8 years!).
Again, thanks for your replies. Please don't be offended if you feel I'm disregarding your advice, which is kind and sensible

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Fred