Could he participate in an online class? He would still get support, and it may not matter that he is significantly younger than other students. They might not even know.
I'm eyeing the AOPS classes interestedly for later, since as you say it would be good to put him in an environment where age isn't obvious! Right now, although they have just introduced some new times that come in our evening (unlike most of them, which are in US evening and our middle of the night!), it wouldn't work logistically - that's supper and bed time. (He might also have trouble keeping up, as his typing isn't there yet, but we're working on that.)
AFAIK there is nothing like that during UK daytime.
