Where are you located? I know many surgeons will do surgery at 16 or even younger with parental permission.
Also, sometimes, as mentioned above, binding can be comforting for those with autism spectrum disorder, as the compression is soothing. Some children with ASD actually wear compression vests for comfort! But I understand for others it is quite the opposite.
I am so sorry your son went through puberty so early, that's really unfortunate. While blockers and T won't remove the breast tissue he has already developed, they will prevent more from growing.
Have you discussed his situation with the school he will be going to? Maybe that would help ease his transition into high school.
It sounds like (from his lack of interest in support groups) that he doesn't want to be reminded of or deal with the fact that he is trans, and just wants to be a regular kid. Maybe playing boys sports or hanging out with cis gender boys may help him feel more at home than associating with other trans kids?
While we, as adults can see benefit in him meeting others like him, he may not be ready and may just wish to ignore his trans status for now. Not sure, everyone copes differently.
Best of luck, your son is lucky to have such a supportive and active mother.