Let him know that even if it is 'just one more year' he doesn't have to endure it if he'd rather try out somewhere else. It's important to teach our children resilience but it's also important to teach them they have the power and freedom to leave a situation that is destroying them.
Are you in a grammar area? Are there academic private schools that offer full bursaries? He needs to understand that there are schools where not being sporty and being 'geeky' is totally normal and acceptable.
Meanwhile, indulge in every geek club activity you can find. Warhammer club, science summer schools, chess club, Lego club, music clubs if he plays an instrument, choral if he likes to sing. Ensure he understands that who he is and what sparks him is 100% normal. It may not be as common as being a sporty bully in the local school but that doesn't make him abnormal and it certainly does make him a more desirable human long term.
It's no comfort to them (but may be to you) that I think children have an in built sense of who will surpass them long term and they are jealous of it and can only express that jealousy through aggression. The brainier ones often get bullied. Or the lateral thinkers. The non-sheep. So many of the world's most successful people had no friends in primary school, few in secondary, good friends in uni and life long friends thereafter. The clans in primary school often find it works the other way round.
Childhood sucks for children who don't fit into the herd easily. Just keep letting him know he is 100% fine as he is. That they can't see that is their problem. That their judgement of him in no way defines who he is. He might not believe you at all right now, but he still needs to hear it from you with your total conviction.