PTSD can be complicated. Here's the breakdown: Someone has a bad experience... the result of having that bad experience can effect them in all kinds of ways. They may have reactions that they aren't able to control; things that the brain does automatically according to proper or improper function (Scientists know a lot about the brain, but they are a long, long way from knowing exactly why and how it functions the way it does). It may cause them to be physically sick for reasons they can't control. Their dreams may haunt them. They may not be able to control how they react in certain situations.
It's similar to being in a relationship with someone who has a physical disability-and, yes, that would mean he has two disabilities. But, there is hope that the PTSD may fade away.
The obvious therapy is love. Love who they are. Love who they want to be. Share your sincere feelings with him. Share who you want to be.
Certain medicines may help, too. One that I recommend is CBD oil.
And, above all else, you've gotta decide whether you want to be with him or not.