I would be really uncomfortable with unsolicited advice, especially from someone I'm not close with. Since he's stuttered that long and been through speech therapy before, I'm guessing he's considered it multiple times over the years. Whether you go to speech therapy is a really personal decision, and I spent like 10 years in it and didn't improve. Whether he's going or not, that decision probably has a lot of factors and he might not want to talk about his personal medical decisions with you. I get that you mean well, but I would definitely advise against bringing this up, given the context you've shared here.