I’m a speech therapist and I often get 12, 13, 14 year old boys who come in for stuttering therapy because their parents want them to. It’s not really effective if they themselves aren’t interested. I’ll have serious discussion with them about if they themselves want to be there and if they do I’m happy to give them some tools and strategies, but if they don’t then I’ll just let them know that there are tools and strategies that exist and if they ever want to return, they can. I am usually also pretty up front that their stutter will likely never go away completely, even with speech therapy, but we can work on some things that will make them feel more empowered and may help reduce the stuttering a bit. I would back off and follow his lead (as the others have recommended)