Maybe he just has given up on the methods he learned in therapy. Could be that his speech tools don't work for him anymore, or he doesn't see a benefit in reducing his stuttering by replacing it with the new abnormalty that comes wiht the speech tool. Anyway, there are two mayor therapy approches to stuttering that have a good chance to help at least a little bit. Namely "Fluency Chanping" and "Stuttering Modification". You could offer him to inform you about these and search for a good therapist. But please don't skyrocket your expectations on results. It's a hard way to gain more control over something so automatic like speaking and stuttering.