OP here - just to add on this - his middle school is very large, and the social skills group is limited to kids on an IEP (he is on a fairly extensive 504, but not an IEP) or kids who teachers have identified as having difficulty connecting at school. But he would not be screened into the second group because he has people to sit with and talk with at school. It just never transitions into out of school friendships, except for his one female friend. I have tried encouraging him to make other social connections through her, but his success with that has been limited to more people to talk to at school, never outside.
If you can afford it, try a PEERS group. It literally teaches kids the steps of building friendships, planning a get together/hangout, solving conflicts, etc. It lists the steps that come naturally to us NT people but kids w/ attention or social difficulties can miss easily.
Thank you - I have read about PEERS and it sounds promising, but I cannot find a group that meets near my home (in Fairlington). I plan to try to look for one that meets this summer, when I will not have to coordinate with DS's band and school sports practices. I don't want him to drop either of those for the time being, as they are connections to kids at school.