L.D.
I would ask his teachers what they think. What you are seeing is just a snapshot of his day and it maybe the case that he is social with the other kids, just not as often or as enthusiastically as you may with he would be. If the teachers give you a favorable report or don't seems to be overly concerned about his friendships at school (or lack of them), then you may have a child who is a bit more reserved than the other kids and probably would do better with more access to 1:1 playdates as well as having his share of alone time as well.