Hi ChloeX, that's great that he is initiating conversation with you. I wanted to share my experience some in hopes that you may find use of it. Regrettably, I used to expected too much validation from my last boyfriend(due to ptsd) who has since passed away. I used to do what your sufferer does...break up with him and isolate myself for months.
I sympathize with you and your sufferer. I hope he is seeing a T or getting some help, because it sounds like he is really hurting. And the worst part about those retreats from who you love is that it's the most alone you could ever feel and all you want is for the person you ran from to come to you. In my current, healthy relationship I isolate myself but not in the same extreme way. And it helps that I get constant affirmation of his love. From how I see it, it sounds like your sufferer is so crazy in love with you that it brings up uncomfortable feelings he doesn't know how to deal with. For me, it's come with much time and reflection with a therapist to separate my ptsd symptoms and not let them affect my relationship. I know that when I feel like my guy does not love me, it's some deep part of me that needs validation of love which he cannot fill other than holding me and telling me it's okay. By him always showing me that he cares and keeping open arms helps that part of me that used to run away feel less afraid and I cave to him instead of isolating. I hope you can be there for him, but don't forget about yourself either. He needs to work with a therapist and be around those who love him. It sounds like your heart is pretty big, he is lucky:)