Be honest. if you have questions for him, ask. Try not to be overly helpful. If you wouldn't interrupt or finish someone else's sentences, don't do that for / to him. But be yourself. Sounds like that's the person he's into. Many of us feel some amount of shame about our stutter. It can be a very private matter. One that we should talk about and which we typically don't. If he wants to talk about it, just listen.