Grinity-
Wow, this is a big decision. I think you need to sit down with DS and DH and really look at the school situation from every angle. I don't feel like I know your DS well, but it seems to me that he would not ask to go back to public if he hadn't thought about it carefully.
FWIW, My HG+ little brother went from private school to public with hopes of making it socially as a athlete. His private school was super competitive, and he was a good athlete but a late bloomer and a head shorter than most of the other guys in grades 9 and 10. He played varsity lacrosse but didn't make the hockey team. It was harder for him to connect with the public school kids, and it wasn't that we were wealthy- we were most definitely not. But he was a very sensitive kid and had a lot of anxiety and perfectionism issues that made it tough for him to feel comfortable in the much larger public school. I don't know if this example means anything to you, but my mother has always felt that moving him was a big mistake. It was as if his expectations for himself got lowered as he struggled more with
the social stuff.
Good luck!