This is very difficult. We have had the most success with outside activities. A club at school would be great to start, but there may be other opportunities in your area that your son would enjoy trying. 4H, scouts, math clubs, others were already mentioned. I have noticed boys who hang out at the library often are very bright and quirky.
You say your son is not interested in sports, but there are some things out there that he might like that would give him opportunities to invite a classroom kid along and have something in common, like frisbee golf (courses might be set up in local parks), even just mini golf or hitting balls.
I would suggest also asking the teacher which kids at school seem to get along with your son. At this age it is not too late to set up play dates for him.
