My husband stutters, and the best thing his parents (who are the foster parents he was placed with as a young teen but they are his parents in every way) did for him was to not try to get him to stop stuttering but to just try to make things easier for him as someone who stutters. I’m not saying therapy might not be helpful (although I don’t know that I’d push it with an adult child who has indicated they aren’t interested), but if you are bringing it up in a way that gets across that you want him to stop stuttering or to stutter less that probably won’t help him and honestly might make him feel like you care more about how he talk than what he’s saying.