Kinda torn on this one. First of all, (assuming you are young) there's a good chance he's thrilled just to have someone listening to him at all. I know this doesn't really relate to your question but I feel like it's something you'd like to know. Personally I think you should just treat him (seemingly) normally, but there are subtle things you can do to convey to him that you care about what he has to say. If he is in the middle of a stutter, make sure to keep your attention on him, don't get distracted by other things. If someone else in the conversation cuts him off, continue to focus on him instead, or maybe even speak up and tell that person that he wasn't finished speaking. If you really want to outright tell him, I would first just show some genuine interest in the condition itself. Ask him some questions about it, like "if you don't mind me asking, I'm just curious, how long have you stuttered?" And things like that. Personally I really enjoy talking to people about stuttering, but naturally people are nervous to ask so I don't get to do it very often. You can then use that conversation to lead into telling him that he can feel comfortable talking to you and that you don't mind. I feel like this would give a much better chance of him not feeling like you're pitying him.