How to encourage my 13 yr old to be with friends?

My son doesnt seem to want to have friends. I am really baffeled at him. He is really shy, mature for his age, and a good looking kid, and plays sports, he is on a lot of teams and goes out plays hard and just comes home. He never tries to become friends with anyone. He is not bullied, I am pretty sure, as I am friends with lots of the moms and they ask me why he turns down invites. I have asked him why he doesnt want friends and he just say he doesnt know. He does have one or two friends but they dont go to his school. So he seldom sees them. I dont know how to encourage him to have friends. I am not trying tomake him be mr popular, but having friends teaches important social skills of getting along. He likes to hang out with his brother who is younger when he is at home.. I dont know what to do? He doesnt seem depressed, he has alot of fun at home...Any suggestions?