If he doesnt know you stutter then you could say something like "hey man i stutter so that's why i talk like that" and most of the time they Will appoligize and it's all good and maybe they ask you something about your stutter and then you van have a little conversation about it and i find it that if you talk about People about your stutter it Will make you automatically calmer and stutter less around them, and they know so they don't make comments like that anymore