With his bd, he sounds just about one year younger than my dd13 (she turned 13 in early Sept). She's in 9th this year and no one seems to know that she is younger. I'd definitely push for at least more subject acceleration. Unfortunately it sounds like your high school doesn't have to offer what he socially needs given the general level you describe. Do you have any options for extracurricular programming (summer camps, weekend programs, etc.) where he could find a peer group if he isn't going to get it at school?
I'm thinking that, if you could get him accelerated more in school so he'd be at least a little challenged, and then try to meet the need for peers externally from school, that might be the best you'll get for now.
