New friend has a stutter or speech impediment. How can I help?

New friend has a stutter or speech impediment. How can I help? In a circle of friends I have (arranged through meetup.com), there's a new person who's been coming to the meetings who seems to have a stutter. He also speaks very quietly, and it can be hard to understand him sometimes. How can I best hold a conversation with him? Should I discuss the stutter and try to help by guessing what he's saying? Should I be patient and wait for him to complete each sentence, even if it takes longer than usual?