In my experience it comes down to people being sure someone is going to leave them because they can't deal with their 'issues', if you just stay and support emotionally, eventually he'll understand that you aren't so superficial.
As for the nightmares, it seems like over time they start to be less dominant. But, like others would say, they don't want to remember them, and spilling it to their significant other can cause other problems, no matter how well meaning you are.
Like the person before me said, maybe encourage without pushing-for him to seek professional guidance.