I think you may be projecting a bit… feeling how you would if you had his stutter. I (mild stutter mostly controlled) feel this a lot when around my brother whose stutter is… a larger issue. I take deep breaths to manage my own anxiety when he’s stuck on a word or struggles through a difficult sentence. My anxiety is about my stutter, fear, shame, etc. That’s all mine to deal with. Not my brother’s. I know finishing his sentence isn’t helping either of us and I need to focus on being present with him. With him, focus on being present. If he were any other person without a stutter and you couldn’t understand what they said, do that. If concerned, ask him how he feels when you ask him to repeat himself. That’s a tool called “Checking the Facts.” You think something is a certain way so you have to check in with others to either confirm or deny. And good on you for reaching out for advice and sharing your experience. I’d also encourage you to eventually share it with other non-stutterers. I think most people don’t know what to do when they encounter someone who stutters besides making a joke (after seeing it on tv/movies/YouTube as a joke). Your experience is important for all of them.