@Compsearch - answer is 'yes and no' I think - isn't it always?
Probably, because you're musical yourself and your son does so much outside it won't matter much. In school music will likely always be at a level that doesn't challenge him in any state school but again that won't matter.
To get a musical cohort though you could look at state schools that have music scholarships - Kingsdale, Haberdasher's, somewhere called Saint Cecilia's I think - some catholic schools. The super selective grammars will be absolutely stuffed with Grade 8 kids who do music on the side but probably excel in STEM (because that is how their selection tests work).
But, as I say, with encouragement outside it doesn't matter. UNLESS they stop doing music GCSE and then A Level (which is what happened to us). A Level doesn't really matter as you can move for Sixth Form, but DD2 really wanted to do the GCSE. But that's because she probably wants to go to a high level uni to do music, it wouldn't have mattered for conservatoire because that's just on playing - you don't need the rest of it.
Two children from DD1's London school have gone off to conservatoire in the last two years - their music A level results won't reflect their ability in music at all (and the A Level in music won't run for DD2's year) but playing is done out of school anyway.
So pick a school with a decent music cohort, perhaps music places, and just be prepared that he'll still be way above everyone else unless its a grammar I guess is the answer.
Sorry, that's an essay!
But I was pondering it this weekend as DD2 went off and had a (successful) chat and sing with a big Oxbridge college for potential choral scholarships. She could never have done that from her state school unless she'd also been a chorister, and we had never even heard of people being a chorister so that wouldn't have happened. Even though she's an NYC member, she just wouldn't have had the experience of sacred choral singing, nor the confidence.
As it was, the psalm singing, sight reading etc didn't phase her after two years of her new school choirs - helpful that hanging around a cathedral all the time makes it all less intimidating too.
When I went to uni, I'd only done Les Miserables medlies in my school choir and had to learn it all at my much less prestigious and non-auditioning chapel choice. Psalms still panic me!