Stop being friends with him. If his stutter troubles you on such a level, just leave him be. Like other people have said on this sub it’s amazing that he has the confidence to not care about it, something that a lot of us struggle to do. I promise you he does know that he has a stutter and you telling him to think about what he has to say is not beneficial whatsoever, frankly it’s extremely ignorant and disrespectful. Either be supportive, let him finish his sentences and treat him like a normal person, or just don’t be friends with him. I apologise if this came across as rude but it’s really upsetting to see your reaction. He’s trying so hard to speak, something that may come to you naturally is actually daily struggle for us. And if you can’t see that then idk what to tell you.