I never done therapy. But doing theatre and speech team in high school and college helped, as it’s in a way exposure therapy. Also, being a teacher assistant in college helped. What helps me is emphasizing words. If I can emphasize a word, it allows me to prepare myself for the next word that I choose to say. Speaking a lot too helps, because it allows me to recognize my speech pattern, making speaking more familiar to me. Note: this is all just anecdotal, so it may or may not help for your son. You already being concerned for him already shows how great parents you guys are. Just keep trying your best, as the worst thing is about stuttering is the psychological effect it has on the stutterer.