One thing that might help: Quality not quantity. If he's having 4 good hours a week with other kids that's SO MUCH better than 40 bad hours. If he can start getting positive experiences it will go a long way to helping him be open to more and more as the years go on. For that first year I wouldn't push it too hard or stress that it all has to get fixed right this minute. The mentors/teachers are great and if you can get a couple of other one on one things that's good too. If you are going to think about a group class I might look for something that would be helpful with sensory organization. Some common suggestions would be karate, swimming or horseback riding, but it may be better to wait a year.
Once things settle in a bit for him, one thing we've found that works well is to organize small group activities - could be even just one or two other kids. If you will do the planning and get together supplies and/or a teacher if needed, I bet you will have no trouble finding another kid who wants to be involved.
