Speaking from experience here--I used to dread my birthday because my mom would always want me to invite "the little girls in my class" over for a party. My issues were different from your son's but I had a similar feeling that I was being tortured when my mom just wanted me to have a good time at my birthday party like other kids do. I was relieved when I turned twelve and she agreed that we could just go out to dinner as a family.
It seems to me that your son's social development is fine. He has friends who reciprocate the friendship. I think it is fine to cultivate those relationships and not worry about age peers. Think about the people your son will be interacting with in his possible future careers--would he fit in? It doesn't sound like it will be a problem
By the way, even though I had a very hard time making friends as a kid, I enjoyed college and have had plenty of friends since then.
Cathy
