I mainly wanted to tell you something very, very important: Thank you. Thank you for caring and for asking. I would like to add my own opinion (or how I would want to be treated) but my answer might be a little traumatized, just keep that in mind lol :D If he doesn't know that you know, give him him time and let it come naturally. If you see him run into blocks and/or get very nervous, anxious or helpless, try gently asking about it. When When I'm nervous or running into a block, I'm already sweating and beating myself up and I would really appreciate some kind and understanding words like "don't worry, it's ok" or "take your time, please" If he knows that you know and you both feel comfortable about it, ask him about it, maybe even show interest. A simple "have you always stuttered?" or "is it difficult living with a stutter?" can show that you care and are not insensitively assuming something or pushing him into a direction (note: yes, stuttering is difficult in my opinion but it's a very easy and broad question and it shows that you want to learn about it.) One more thing: Those are just my spontaneous ideas. That's it. Compassion and authenticity are key. Show him you care (through intention, tone, seriousness, etc.) and don't want to make fun of him or whatever. And if you're comfortable, ask him how you can support him. Feel free to ask if something's unclear or if you have another question :D Cheers