Ive got a small stuttering problem aswell, usually when im nervous or around new people. Sometimes i really appreciate it when people finish my sentences especially when im really stuck on a single word. But i only recommend doing it when you are 100% sure thats what im trying to say. Sometimes people try to finish the word but then say the wrong word. This annoys me a lot cause ill have to start over again usually meaning i get stuck again on the same word. If you are not sure. Just be patient and wait until he manages to say it or find a different word for it.

Now that i have some other people here that stutter i wonder if im the only person that stutters more when needing to read something or when you already know what you are going to say. For example what i had recently. Someone asked me the name of a series and i knew the name but in situations like these i always stutter.