You don't say how old your son is, but I'm guessing 4-6, in which case, you still have a lot of influence... and that's a good thing. I admire your boy's self-awareness. He knows his comfort zone and that will serve him well in the years to come.

One solution is to find a friend who shares his interests and his temperament. Are there sons of your friends who are less of the "rough and tumble" kind of boy? Might you set up a play-date with that mom and son? I'd also suggest that you talk with the teacher. Teachers are very insightful about the personalities and temperaments of their students. Ask the teacher who (s)he thinks would be a good friendship match for your son. Ask if she could try pairing the two of them together... that will naturally foster friendship.

One final suggestion: Let your son's interests guide you in choosing some outside of school activities for him via, parks and Rec. Dept. classes, Boys & Girls Clubs, the Y, Scouts, etc. Most communities have outside of school opportunities for kids of all ages learn, have fun, and make friends.

I hope this helps!