I'm really unsure. I've met stutterers who really dislike people finishing their words whereas I actually don't mind it at all. Depending on how close you are to him, I would just ask him privately. No need to do it infront of the entire group, just approach him and be like "Hey xxxx, I was just wondering. I have many friends that stutter and some of them like some help finishing words and other people hate it. I am wondering if you want people to finish your words or blah blah." I don't really know, but maybe ask something along the lines of that.