Okay, that detail helps. I don't see anything worrisome or immature in that description. One thing I've noticed with kids that age is sometimes kids who have spent more time with adults and have been socially coached more are actually a bit TOO mature for the age group. I'm wondering if that's the case with your son. Most kids that age will not formally ask if they can play and may not answer if someone else does. Kids are more likely to play near each other for a bit and then join in by running in the same direction, saying "wanna race?" or something like that. You might want to take a look at the book Good Friends are Hard to Find by Fred Frankel. It is a book for parents on how to help their kids learn to join play and get along with other kids.
http://www.amazon.com/Good-Friends-Are-Hard-Find/dp/096220367XWhat other specifics are you worried about besides the reaction from other kids?
