That's his choice. It's his pain and his decision. Some people choose therapy easily, others don't. Some do awesome in therapy, others don't. Some people get better in therapy, others don't. We are all different and we all deal with things differently. If he's not ready for ther therapy, then you have to accept that. It's his choice, just like it's your choice to stay in the relationship or to move on. You can't force him, convince him or push him (it only make it worse).
When and if he's ready it will be his choice and he will seek counseling but until then, you can love him for who he is (faults and all) or you can make a decision that this crosses the boundries that you have set for your life and move on.
It doesn't make him wrong for not seeking therapy. It doesn't make you wrong for standing by him or walking away. Loving someone who has PTSD is not and never will be an easy life (even if they are in therapy and/or meds). PTSD triggers show up whenever they want to and steals the person you love for a while. It never goes away. You have good days, weeks, or months and you have bad days, weeks or months.
Therapy can help but it's not a cure. You will always have to fight PTSD from stealing the one you love. It's reality. It will never go away and you need to realize that.
Therapy is not a cure all and sometimes it makes things a whole lot worse before it gets better. It also takes years of therapy to really get somewhere.
So what I'm trying to say is that even if he went to therapy your relationship would not get instantly better. It most likely would get worse for a while because therapy is hard. It brings up trauma that the sufferer has tried to suppress to survive. That's why most of us supporters are in therapy as well because we have to learn to deal with PTSD as an invader of our relationship.