Be honest with him.
One of my friends (who really DOES know how I think) told I guy I used to go out with, the thing he had to realize about me is that you have to "prove up" everyday. I don't think that's literally true, but it's kind of true. I ALWAYS wonder if I can trust someone. I'm always looking for evidence that I can't. That's not about them, it's about me. What I appreciate is someone who approaches things in a simple, uncomplicated, straightforward manner. When I get to believing I can count on that approach, then I can quit working quite so hard to look for "real motives". And that's a HUGE relief. If I can count on someone to tell me exactly what they are thinking and how they feel about stuff, I appreciate it. If they expect me to read their mind AT ALL........ I'm terrible at it, I KNOW I'm terrible at it, I won't do it, I don't want to do it....... Get what I mean? I can and will forgive someone who decides they've made a mistake. I make LOTS of mistakes myself, I'd better be willing to forgive others. What is just "too hard" is trying to sort stuff out without enough information.
You sound like you're being pretty hard on yourself. There is no reason you "should have known" anything about him he didn't tell you. There is also no reason you should have known anything about a subject, PTSD or anything else, that you were encountering for the first time. We're all human. We all do things we wish we'd done better. We all do things we WOULD have done better, if we'd known more. No one ought to get hung for that. You pick yourself up, you explain what happened to anyone worth explaining too, and you drive on.
I hope things work well for both of you. But, now that I think about it, there are also times when a person needs to make a journey alone. Right now, I'm not sure I'd want a SO in my life. If it were to happen, I guess I'd deal with it, but there can be lots of potential for "collateral damage" and sometimes it's just easier not to have to worry about that for a time. Does that make any sense?