Personally, like a couple of people here, I don't mind when people finish a word that I get stuck on. I usually laugh when that happens and naturally say a quick sorry, and carry on. Some people are more self conscious and aware of their stammer though and can get annoyed if you finish a word. As everyone has said, patience is probably best, but most important, to me, is to really listen an take in what he says and try and take as much meaning as you can from it. If he's anything like me, he won't speak in a group unless he feels he has to and even then, I wouldn't say it in as much depth as I wanted. But when I say something, an maybe stammer a couple of times and either they say "what?" Or just carry on like what I said wasn't that important, that can suck. As long as he feels heard, he should be cool at the end of each struggle. But that's just from my experience and how I would take it and again, like said, everyone is different. Maybe even talk to him about it in private next time