You have to decide what you will and will not tolerate. Then stick to it.
A big boundary that supporters talk about setting is with their sufferer's lashing-out type behavior. When the stress-cup overflows, supporters are usually the closest in proximity and deal with the brunt of this kind of reaction. It is not fair, and you should not have to deal with it. It will eventually break your spirit and make you feel like dirt.
When my vet lashes out at me, I simply say "I love you, but I will not talk to you while you are treating me this way.Call me when you calm down and want to actually talk." A lot of times I put my hand up in the "stop" gesture. Then I remove myself from the situation. No exceptions. I do this every single time he starts in on me in an angry or aggressive way. That is my boundary. I will not tolerate lashing out behavior. He can be angry and nasty all he wants, I just refuse to stand there and be a target. He will calm down, call me and apologize... then I go back and we have a much calmer discussion about what was bothering him.
This has done two things for our relationship. Firstly, we don't have a lot of arguments that get to the "earth shattering" level. We don't get to the point where we say things we can't take back, or that will take days to recover from. Secondly, he has learned that if he wants to discuss things with me that irritate him, he needs to cool his jets. He doesn't lash out nearly as much as he used to. He also knows that I recognize this as a PTSD thing and do not think he is a giant a-hole. When he is feeling well he is a very very nice person.
Setting a boundary is as much action as words. If you tell him that you can not tolerate a behavior, then you cannot tolerate it.