Is he unhappy with his current friendships, or his social life. Is it possible that he at 17 is trying to stay out of social circles that are doing things he just doesn't want to get into. He may be that wonderful kid who doesn't want to get into the teen cycle or drinking, sex, etc. which is good judgment in my opinion. If your son is happy and just shy then I would let him go. If you think he has social anxiety which is completely different then I would consider getting him into therapy. But, trust me there is a difference. If he is happy and just not as social as you would like him to be then I would leave him be. Trust me I lived through the teen years there is a lot going on our there, he is the one who is exposed to it in school and that may be enough for him. I wouldn't push him and let him go at his own pace. I am an extremely social and outgoing person and my husband is not with new people. I am the person who will talk to the people in the elevator and my husband will see someone he knew from high school and not be assertive enough to greet them. We are all different. My kids would always say to me "Mom why do you talk to everyone" LOL!! Like I said unless he is completely anxious about being social then I would let him be. Teenagers are not always forthcoming with things we want them to do, they have to do them at their pace and when they feel they want to. The only thing I would insist on is that he is respectful when he meets someone new, but I would not push him in those situations. Good luck!!