Op, what sort of school is it, in terms of atmosphere and HT?
My 9 year old goes to private school now (started September), and it was a bit of a rude awakening from the "it is all about taking part" ethos of the village CoE primary, to "you are not good enough to be picked" at the new school.
Reality is that he has been put in the B team, but he is quite pleased to be chosen, at all. When the A team goalie broke his leg, DS was given a chance to replace him in the A team. He did o.k.-ish. He has his first rugby match this week (B team) and is excited (I am scared, it is contact rugby). The sports teacher tries to give everyone a chance at least once, and is also very approachable and sympathetic (it was one of the things I liked about the school). Could you approach your teacher?
Anyway, I felt worried about all this, but interestingly DS is very philosophical. he says the other boys are just better, and he is good at other things (like karate, which he does outside school). I am more upset about him being left on the bench than he is. So I have decided to "let it go" and leave my anxious mum feelings at home.
So I think you hit the nail on the head that sport has an influence on self esteem, but it doesn't have to be in school. We do a few activities outside school. Would that be an idea? DS seems to get lots of self esteem from his Sensei at karate, and it is something he is good at. Better than team ball sports. Maybe try judo, karate, tennis or climbing?
also bear in mind, your DS will not be the only one, it should be about half the class... And all the A-team do sport non-stop at the weekend as well, so no wonder they are better, fair do's really...