In my experience, the people I stutter around the least are the people that I am the most comfortable around. There is a huge drop in my stutter just within the first few minutes of talking to someone new if they seem like casual and friendly people. I'm not trying to imply that you're not casual or friendly, but people who try to make conversation I find are far easier to speak to. Maybe that's just what i define as casual and friendly. I'd intentionally ask him questions that aren't yes or no questions, just to force him into discussing the topics at hand and when he inevitably stutters, be patient and don't acknowledge it unless he does. The less of a deal you make of it, the more comfortable I imagine he'll be. Find something that he's interested in, maybe, strike up a casual conversation for a short minute or two just to get him talking.