Search for a psychologist who has expertise in and treats selective mutism. It is treatable, but you are right that just bringing him in for traditional talk therapy is not going to work, because he won't talk to the therapist. You should definitely read about the treatments in the linked wikipedia article, and then maybe read up some more on treatments that sound promising to you. But you need to do something, and do it soon, because the longer this goes on, the tougher it will be to treat.