Talk to him. I see all these people giving advice on what they liked, and what worked for them. But every stutterer is different. Talk to him, ask him what he needs from his surroundings. For some people that is waiting when they speak, not interfering, for other it’s speech therapy or someone else to go to. When i was your sons age I had the same problem. My parents wanted to help me by saying that i should relax, should talk slower, etc. It made me so mad and irritable, it was really distracting when speaking and made me feel like i was failing, that I couldn’t do it right. You have to go trough your stutter, experience it, get to know it, be friends with it and when you are ready you know how to handle it. I said i wanted to stop speech therapy, and it helped a lot. I wasn’t as anxious anymore and wasn’t afraid to disappoint my parents by failing to speak well. When i was older I said I wanted to try speech therapy again and we went to two different people. Didn’t really do anything lol but that also depends on the person and the therapist . So yeah, my advice is talk to him, ask him what he needs from you and support him in a way that makes him feel supported. Maybe you’re already doing that but you can’t know until you speak to him yourself.