It was mature and courageous of him to consider possible solutions to a significant problem in his life and then make this request. I don't really think it's fair to call him immature for being deeply affected by prolonged bullying and exclusion. Nor is it fair to say he's not brave because he is nor prepared to assault someone.
Resilience comes from having a secure foundation of feeling safe, loved, acceptable and included, as well as being supported to face and overcome reasonable challenges so that you learn the skills to do so and also the knowledge that you are capable.
It does not mean being left to try and 'tough out' unacceptable situations that should not be occurring. To call that 'resilience' is an abuse of the word.
The school must address the bullying as an urgent priority.
Separately, in your role as parent, it sounds like he would benefit from activities where he doesn't feel held back by his size and can experience feeling good about himself and successful for the person he is. Rather than always feeling inferior and on the outside. Never quite good enough.
Essentially, the opportunity to feel acceptable just the way he is, rather than the mindset of having to tolerate being inferior which is what you're describing.
Comparing him to siblings who you feel have coped better is actually exacerbating that - he is a different person, he is having different experiences, and therefore different struggles That doesn't make him a failure or weaker, just a different child with different needs to be able to cope and thrive. I am sure you don't make the comparisons to hurt or diminish him, but because you want him to be able to cope as well as them, however it does seem to be creating a sense of judgement that he will undoubtedly pick up on and fuel his negative self-esteem.
You mention relatives who are jockeys. That clearly plays to their strengths and I can imagine it brought a sense of satisfaction and acceptableness. They were able to achieve and succeed because of their size rather than despite their size or having to compensate for it.
Whilst becoming a jockey isn't really the right answer for a ten year old(!), I really do think that if you could put some serious consideration into finding and creating opportunities for him to have something where his size is perfectly fine it would really improve life for him in the long term (so he can experience a straight "you're good at this" , as opposed to the constant "you're good at this for a small person or "you can't do this because you're so small" ).
If you could implement both strands - school eliminating the bullying and othering, and you finding the right activities to build him up and let him see that his size is one quality among many that he has not a fault or a flaw - it could transform how he feels and his ability to feel good about himself.
Once he feels that he is acceptable just the way he is and has positive self-esteem, that in turn will boost his ability to cope with the odd comment or disappointing moment of not being able to do something because of his height. Effectively, it will create the resilience you of course wish him to have.
I would be very concerned that keeping him back a year instead of addressing any of those serious underlying problems would just reinforce his negative self-esteem and make the problem worse in the long run. It is a difficult situation I can see that you only want the best for him.