Kudos to you for being proactive and thoughtful on how to best connect with him. Patience is key - as regGeo said below, don't finish his sentences (even when done by the most well-intentioned person... it's still a pain). DO, however, use your speech as a model and focus on articulating your words carefully. It helps me at least when I'm in a rut and tripping over myself. Last point - in one conversation I had with a banker, after we made a little small talk, he said something like "I noticed your stutter... my mentor had a stutter and..." - we had just started talking, but it was nice to have him acknowledge it. Some stutterers aren't quite at the level of announcing their stutter to the world (even if it is noticeable), so you being open to acknowledge and discuss it might help put him at ease.