I'm in my 30s and stuttered pretty bad as a kid and teenager. Look into therapy for him. Check out local colleges and see if they have any speech therapy programs, I know one of my local ones does. Have him talk and practice talking in a very stress free environment if you can. Maybe in a car ride get him to talk about some favorite topics of his. Practicing and just talking will help. I noticed over the years my stutter would be worse the more stressed or anxious I was.