16 year old has no friends

My son is 16 and has no friends. His dad always calls him a man of few words but it's worse than that. He doesn't talk much at all and when he's in situations where he needs to talk he can never articulate what he wants to say. He will stutter really bad and eventually just give up. Two years ago he also got really into heavy metal so he dresses in all black now and a lot of people find him kind of scary. I coach girls soccer at the high school and went off on a mom who referred to him as a future school shooter. We had a moment 6 months ago where he gave me permission to read his diary and in it he's writing about having things to say. Just never being able to get the words out. So he usually nods. Shakes his head or answers with a few words. I'm not sure if therapy would help because how can talk therapy work if he won't talk?