Like somebody mentioned earlier, just treating him like a normal person will do wonders as it will alleviate stress that may exacerbate whatever stuttering habits he has. I think a lot of what stuttering people would wish non-stuttering people would understand (aside from common sense ie mocking, etc) is wholly on a case-by-case basis which he may choose to share with you or he may have already expressed those to you. Personally, my SO understands to not complete my sentences or to not do things that would relieve me of that stress of stuttering (ordering at a restaurant, phone calls, etc). Other things I find are kind of handled as they come -- for example, my SO complimented me on a few lines I said with decent fluency. I don't want to say it was completely out of the blue as we had a talk about my stuttering earlier in the week, but it was entirely random for them to say as they had never said it before in our length of our relationship. I told them to not isolate and compliment as it kind of made me feel really anxious and vulnerable to and they agreed, and here we are. I think that as time goes on and I get further into my self-therapy things like that won't bother me so we can re-evaluate that as a couple when the time comes. To me, it really comes down to communication and being vulnerable with each other about matters like these.