Some people like it, some people don't. Personally, I hate it. To me it seems like I don't know how to talk. I stutter, and it's frustrating, I get it. But I'm trying to get better, and as I know the other person I eventually stutter less. So if someone doesn't want to hear me, then I will never get better, and probably get worse because it's confortable not to talk. That being said, you should probably ask him. Tell him you noticed his stutter, assure him it doesn't bother you and ask him if he prefers if you finish his sentences or not.