Sports classes, new kids, coaches, practices and "other traditional methods of athletic and social progression" are not the only way to progress socially. He might be doing fine in other venues and other ways. It doesn't have to be sports. It could be academics, music (where do you think rock stars come from? not the football program), acting (ditto), art, computers, or individualized ways to get exercise, such as running, cycling, martial arts, golf or horseback riding. I also, like margy, wondered if you are pushing him on this even just emotionally. If you had a great experience in team sports, it does not necessarily follow that he will. Other things just might be more to his taste. I'd try to find and support those things, since a happy child who feels competence in something is miles ahead of other kids.
Could it be he is just not ready and you are ? Would you be pushing him a bit and this could cause anxiety, maybe an idea is to let it go for the moment, make him aware of the things he could do if he wishes tothen leave it , , he may be overwhelmed. Not all children are sociable and it sounds like a lot going on already, I understand how you would want him to participate in sports and classes,I do think it is very good for all children to participate in sports ..However hes got to want to.... so take a proverbial step back, let it go and see what happens ..Good Luck he may just not be quite ready to be that social ....