It's nice of you that you wanna know all about it to make him comfortable. The most things are written. Stay calm, keep looking him in the eye. And maybe you mention it if he doesn't? For me it's works when I mention it that I stutter and go on with the conversation. (I mean when I meet new people which I know I will see them often in the future) And in the past I often didn't have the guts to speak about my stutter. I liked it if someone else mentioned it, but most of the listeners don't do that. It is tricky... some people who stutter should hate that. But then you can tell him how you think about it, that it doesn't bother you. And THAT will give him more calmness I think.