I wish I could say it sounds like he’s emotionally involved because it sounds like that’s what you really want. Unfortunately, I have to agree with most everyone else. He doesn’t stay because he doesn’t want to, not because it’s too hard for him.
I suspect there’s an easiness between you two. You know each other so well that having sex is comfortable and satisfying. But, because there were problems which lead to a divorce, it sounds like your relationship is just slowly winding down. Because you want a full-fledged relationship, I think you need to get out of your current situation. Only then can you really move on and find someone who wants to spend not just the night with you, but a lifetime! And, there is someone out there for you. You just won’t find him until you make up your mind to stop having sex with your ex. (Of course, you know any new guy would not want to date you while you continue to have relations with your ex). And you really don’t want your ex to make the decision to end your arrangement when he finds someone new. That will really hurt your self esteem.
You sound like a loving and caring person. Take better care of yourself (you deserve it!) and find someone who loves and cares for you.