My son is 16 and has some introvert tendencies. he made Varsity this year as a sophomore, and is struggling with the social aspect of it. None of the upperclassmen have gone out of their way to include him outside of practice/games (like a big group of them go out to eat a lot after practice) and he isnt one to include himself. I know it weighs heavily on him, but its just his personality.
His biggest fear in life is being in a crowd of people where he doesnt know anyone. He literally used to make himself sick...like actually vomit... the night before the first day of the school year. When we would go out to eat, he wouldnt talk to the waiter (until around 10 we finally told him we wouldnt order for him anymore).
He has a few good friends, and around them he is almost comically boisterous. But away from those 4 or 5 kids, he is shy as shy can be.
Its funny because on the field he is an entirely different person. Always has been.
I dont really have any advice, just some commiseration. One thing ive learned about my son is to not try to push things. I offer support where i can. Its hard watching our kids struggle. I know. I was kind of a social misfit in HS and i turned out ok, i guess. They will survive.