I in no way mean to sound rude, but you do realize stuttering isn’t voluntary, right? As a person with a lifelong stutter, I can assure you than telling him to think about he wants to say before starting isn’t going to help him in any way. I have had people tell me this before, and I personally find it extremely offensive. It’s insulting to his intelligence. MANY of us struggle with this in our lives. Many of us have self confidence issues. Many of us have a trouble finding jobs, making friends, or finding a partner. This is an extreme challenge for many of us. If he’s not struggling with it, let him be. Never. EVER. Cut someone off when they’re stuttering. Again, extremely rude and insulting. I’m sorry if I sound angry, I’m trying not to be. I know this is coming from a place of trying to help your fiend, but in my opinion you’re going about it all wrong. He needs support, care and compassion. And that may come in the form of just not saying anything about it. Best of luck to you and your friend.