LostOne-- I don't know. I wish I did but I don't know.
A lot of it will depend on him. For me, I had to get somewhere bad enough that living that way was no longer acceptable to myself. And that was a very very very dark and painful place. To get there I had to push away someone I loved very dearly, I had to believe that I was not worthy of love and happiness, and I had to believe that there was no hope. Paradoxically, I had to believe that there was no hope and yet that there must be some other path-- one that I couldn't see and didn't know but that I HAD to believe existed.
It doesn't sound like your partner has gotten to that breaking point yet. Until he gets to that point, facing reality will be too scary and not worth the pain he knows he feels but is suppressing. He's probably still clinging to the belief that this will blow over, or that you'll do something to fix the relationship or maybe he's just in really deep denial/unawareness. When he's ready, he'll come to realization that he has suffered trauma, that it has profoundly impacted him and that he has to do something about it. Hopefully, he'll also believe that he can do something about it with your support-- but honestly, the closer your relationship with him (especially if you're not married or don't have kids together), the more likely it is that he'll push you away to protect you from him.
What you can do is mentally prepare yourself for this. Believe in your love. Believe that in the moments you stare into his eyes and you see someone else, that is is the PTSD and not him. Take care of yourself because all of this will be stressful and if you are tired / stressed / angry / hopeless, it'll be hard to remember those things. So take time to be outside. Strengthen your relationships with other people. Get a massage or manicure. Allow yourself to get happiness and energy from those things so that you have enough in reserve to face the demons of his PTSD.
Good luck.