PTSD doesn't make you a jerk, but it can certainly make you act like one. It also shouldn't give you a free pass to do how ever you want. If you're going to be in a relationship, that is about "we" and "us", not you & him, if that makes sense. It might be the right relationship, it might not. PTSD will be part of it, but the relationship needs to be evaluated on it's own merit.
From my one perspective, I'm trying to learn how to tell when what I'm feeling/experiencing is "being triggered". I'm trying really hard to sort this out, but sometimes I'm pretty slow. If I was in a relationship, I would appreciate it if my SO would point out what's going on, if they happen to see it first. Because I am who I am, we'd probably have to come up with a light hearted way to do it. For example, once upon a time, I WAS in a good relationship (with the first person to recognize that I have "a problem", in fact). He gave me a goofy nickname and made up a story about the name being for a little wild animal that lived in the woods. They were hard to get to know and very suspicious. Once you got to know them, though, they were a lot of fun. (I realize everyone wouldn't think this was amusing, but I did.) When things started to go south, he would start telling stories about some quirk of that little creature that he perceived was emerging at the moment and needed attention. He started doing this at times when there was nothing "going on", (Did I mention he was a genius?) So, when a problem came up, he had a light hearted, loving, way of making me look at what was going on so I could see for myself that things were out of wack.
That's kind of a long way around to the idea that you need to come up with something that will work for the 2 of you and you can't do it during an argument. You also need to be coming from a place of love, not judgement, hurt, or fear. As far as leaving/not leaving goes having someone storm out of the house can be kind of "triggering" in itself for some people. How about, if you feel you have to leave, saying something like, "I can't continue this discussion right now, I need time to calm down, I'll be back in a hour (or tomorrow, or what ever seems like it will work). Or HE needs to leave, if it's your house. But, put a time limit on it so it's totally obvious that you aren't giving up on him or the relationship, you just think things need to cool off. Granted, it might be HIM that needs to get a grip. I suspect telling him that is going to sound like an accusation and that may or may not be the way to go. He needs to have the same option, to call a time out, but he also has to call a "time in".
As far as him "isolating" goes, how about asking him to put a time limit on that too, for the same reason, so YOU know he's not checking out on the whole relationship, forever. I think you 2 HAVE to be able to talk about this stuff if you're going to make the relationship work. If you can't ever have an adult conversation about things what kind of "relationship" do you actually have?
BTW, the reason I'm trying to figure out what's "being triggered" is, is that once I see it for what it is, I shift gears. I think "Oh, this isn't really a threat at all, it's just the PTSD tricking my brain into going down that road again. I don't have to do that, I can react another way." That let's me change the way I perceive the situation and how I'm reacting. It isn't always EASY to change how I'm reacting, but I can't deal with that until I see what's going on.
I hope that makes sense and sorry this got so long!