I'm am wondering how you help your child "fit in" with other children his/her age?

My DS is in 1st grade and always plays alone at recess and doesn't seem to be making many friends. He has expressed over the summer that he prefers talking to older kids. His school has recess and lunch at by grade level so older kids aren't an option. I think part of his problem fitting in is that his interests aren't quite the same - for example, when I asked him what he did at recess this week, he said he was experimenting with gravity. I don't want him to grow up feeling that he has to hide his interests but I don't see most 1st graders wanting to do gravity experiments at recess. He can be very social and engaging with adults so it's not that he's not social, he just doesn't seem to be able to find common ground with his peers.