As a stutterer a major factor for me is anxiety. I always stutter and I always will but when I’m anxious or super mad etc, my speech gets so much worse. Definitely DO NOT finish sentences for them (unless you’re close with them and have their permission to). All in all just try to make him feel comfortable. Usually talking to smaller groups first helps. Maybe get yourself and one other person they seem comfortable around to hang out with first, then slowly bring in the whole group. Honestly the best advice I can give you is treat them like a human being, not a defect. The fact that you are even asking for advice on how to make him comfortable shows you’re on the right path 😊