I don't know if there's anything you can do. Maybe he saw the new IT movie and got the idea. If he's miraculously gotten over issues in the past, it'll happen again. This is probably just a new thing he's trying. Granted, there is such a thing as adult-onset stutter. It can either result from some kind of trauma - metal or physical - or it could seemingly come out of nowhere. Its uncommon and unlikely, but it happens sometimes.