DS had never played rugby before he started his secondary school, but neither had many other boys. In the first games session the teacher asked the boys to put their hands up if they had played rugby in primary school and divided them into groups on that basis. Those who saw themselves as non-sporty went to a 'non-squad' games group, which meant that 1) they were in a group, fairly big, with similar-minded boys, and 2) they did not have to go to rugby practice after school (although they could if they wanted to) and didn't have matches on Saturdays, which was sort of a blessing on freezing, wet November mornings! Incidentally, some of the 'non-squad' boys decided to start going to rugby practice and ended up going into teams.
Moving up to a secondary school is a big step and can be really unsettling, so it's natural for your DS to feel anxious about fitting in with his new peers. I know he feels very upset, and so do you, but I'm sure all will be fine.
You could tell your DS it's like with the academic side of things - even in a very selective school there will be some boys who are more capable than others and the same applies to sporting abilities. He will find his group of friends he will feel happy with and they will love him to be on their teams.