You have found the right place. Many of the struggles you mention are familiar. Finding one's tribe is very difficult. We have had most success in following interests. It also involves a lot of parental support creating opportunities for socialization. You had mentioned that last year he was with a group that seemed to work well on projects. Have you considered inviting them over for a playdate or creating some sort of afterschool project they could work on together?
Another way to meet others through following interests is to find clubs like history, math, Lego, etc that meet on the weekends. We also find luck with camps such as soccer, Lego, math, engineering, theater, etc.
ETA: You mentioned that your son is ahead in math. Have you considered Epsilon camp? Here is the link:
http://epsiloncamp.org/