This unfortunately is how this illness works and what it looks like. It's apparent that you care very much about him, and its apparent that he is struggling...
I've spent years self medicating with alcohol, and even though I don't use it as much now as I have previously, I know that I will never be able to completely walk away from alcohol as a coping method. I also know that some of my worst behavior was fuelled by alcohol.
It sounds like what he is doing is an attempt (conscious or subconscious, I dont know) to create distance between you two. I do this all the time. ALL the time. With every one. There seems to be a level of intimacy that is intolerable... and things can seemingly be going so well, until I have to ruin it and be back with myself again. It's painful. It's a very painful process and I don't have any answers about it, just some advice:
-What he is doing is not neccessarily because you did something wrong or a sign that he doesn't love you.
-Isolation and withdrawal can be a huge comforter for someone with PTSD when things are getting overwhelming... but it can be almost impossible to pull yourself from it, or to realize how hurtful it is for other people. We just really need space sometimes to work things out alone.
-Sometimes it just isn't personal. It's just about the person with an illness, and sometimes you deal with the person and sometimes you're dealing with the illness. I know personally, I come and I go.
-The best thing you can do is to show patience, understanding, and show him that you care. Reach out respectfully and let him know you're still there and that you just want him to know that you're someone who loves him for who he is and you don't judge him for an illness he never asked to have. However, he has to find some way to deal with his illness...
-Protect yourself and nurture yourself throughout. This is a difficult thing to live with, and it's also difficult to love someone living with it; he didn't ask for this problem and could be trying with distance to "unburden" you from his illness. I do this too.
-If things ever appear to be flying off into something physically abusive, just get out of there. Not excusing behavior, but it's absolutely true that PTSD takes the sufferer into a very primative, reptilian mode of operation and better safe than sorry.
Good luck and I hope you keep us updated. Well wishes for you both... he is blessed to have your care and willingness to try to comprehend his illness.