Don't try to save him or fix him, or even try to save the relationship. My number one recommendation for most supporters is to find support for yourself. That is the best way you can help him.
This forum, a therapist, support groups, friends - the more support you can have around you, the more likely you will be able to really be there for him. This is not because there is anything wrong with you, but because this is going to be a tough road, and you will need all the support you can get.
This may be a far fetched idea - but I have a hunch you are feeling concerned he may want to end the relationship with you. Working on any abandonment fears you have of your own would be a really good idea. Not because there is something wrong with you, but because it might be too much for him to be in a relationship right now and/or he may do that - and it's a good idea to have some support if that does happen.
His negative self image is the message of trauma. Life and death trauma. It takes a lot of time and a lot of patient presence to even begin to counter the weight of the serious on-going trauma he has been through. Try to remember his actions may be about you, and yet they also are not entirely about you. They may be more about the invisible wounds of war.
For many people, it gets worse before it gets better. He may have more symptoms if you try to get emotionally closer with him before he is ready. If he asks for space, honor the request.
You will likely have to endure him thinking he is a horrible person for awhile. It is highly unlikely you can convince him through words that you will be there for him. Instead, just be there for him.
And be there for yourself. Honor your own relational needs. Communicate to him gently but clearly and directly what your needs are in the relationship. Don't make it all about him and his needs. Respect him enough to tell him what you need/want in the relationship too.
