I know it's confusing.
He could have PTSD, maybe not. But if he never dealt with his grief properly, he's definitely got some major depression -- and a problem.

I suffered severe but fortunately acute-ish depression and I know living and being on your own when you are depressed feels like the only way. I hid from the world for 6 months, and as I started healing again in CBT, I ran home to the womb, to my parents and eventually got my sh*t together again. PTSD or depression or both -- he needs treatment, counselling and you're not the woman for the job, it will inevitably break you, especially if he is not ready to be helped and even if he is, you need to be strong -- you don't need to know everything. Once he is healing he will want to look it away anyway.

When you're really, really depressed, peoples energy good or bad tends to zap you -- depression causes you to be a hermit, a recluse, an introvert.
But it is not healthy to spend too much time on your own, that could also be why he find being or living with you makes him feel normal. We are complicated creatures us humans, and even when we think we don't need human contact -- we do. Depression clouds that awareness a lot!

I'd suggest, that if he wants to be in your life, just take him out of the house, for a walk, some chilled time, quality time. And then, if it feels right -- advise he gets some counselling, but don't be his counsellor and don't push him or pressure him to get counselling either, he probably already know she needs it, and he'll do it when he is ready. He may have to hit rock bottom. And it is not your responsibility or in your control to get him there. Just explain that you can't help him, but you can support him if he tries to help himself. Explain you need to protect yourself too or you'll get depressed.

You can't fix anything. You can't control him. I dare say his feelings for you making him feel normal are also connected to his feelings for you, he cares for you deeply, and he feels alien to that feeling, or is fighting it, is uncomfortable with it but knows it's a normal thing to care. He's clearly uncomfortable with normal right now.

I have started a massive post in the introductions forum entitled " I love a very beautiful man with PTSD". Maybe my story can help you.
Feel free to ask questions there, some sufferer friends of mine that I made on here frequent the page regularly too, as well as others who seem to be finding the thread helpful, or at least interesting. So might you.

Be strong and look after yourself -- you can't expect to make a difference until he is ready to make his own.