When a PTSD Sufferer is in isolation mode, continuing to contact them is going to stress them more. Isolation periods are fairly common with PTSD, and we as supporters have to learn to be okay with these and give our loved ones the space they need. When they isolate it is because they can't deal with *it* anymore, be it life, you, stress, a situation, whatever.
You have to decide if you can deal with the isolation in your relationship. It's not going to stop. It's a symptom of the disorder. If you cannot handle it emotionally, then it is not going to be a healthy situation for you or your sufferer.
If you both want the relationship to work, you both have to respect each other's boundaries. You have to give him space, and that means not contacting him if that is his boundary. He has to respect your boundary when it comes to being part of the relationship.
My vet and I have set these boundaries up very plainly. If he needs space, he needs to tell me and not just vanish. I agree to leave him in peace and not contact him. He in return will throw me a text every so often telling me he is alive and well so I don't worry myself sick, and I agreed not to respond to these texts until he is ready. He also knows I will not take a long period of isolation and remain in a relationship with him. Days I can handle, weeks and months are a no go. We have to respect eachother enough to make this happen.