Good luck!
We are having a great time with reading about social skills and then setting small goals for change. And it's working.
I think it's hard for loner type kids to do social skills groups. What is the goal of the group? Is it to make friends? If so, is that a goal he shares?
Is it to improve interaction or to better understand how the social world works? If so, does he see a problem?
Is he preferring the loner world where he doesn't have to deal with people and now you are telling him he must deal with people? Is it a confidence issue?
Even if we see problems, if the kid isn't on board, then the road is rough.
I think I should go back to some of your earlier suggestions. DS is gregarious at school, but never, ever initiates anything with kids his age at home (except for starting forums online, etc.). His BFFs are his 6-year old cousin and 49-year old uncle. And his grandfather.
He does well in these relationships, but school is another ball of wax.
