I highly recommend finding some children's books your son might be interested in and buying a physical and recorded copy. Try to encourage him to read aloud with the recording in private. My stutter hasn't gone away entirely, but this approach did help. There's something strange that happens when a stutterer is speaking in unison with someone else. In my case, if I'm asked to read aloud in front if a room full of people, my stutter is pretty severe. However, if someone were to read along, my stutter goes away completely. If this helps your son, look into delayed auditory feedback devices. Most designated devices are expensive, but there are apps that may help him along. When you notice your son becoming especially frustrated or troubled, remind him that everyone has faults and challenges. His just happens to be out in the open. This realization has helped me curb speech-driven anxiety.