This is just my impressions (I am jumping a little broadly to some conclusions, and could be way off).
I see two things, here. One, he really doesn't seem to be doing enough to manage the complexity of being a poly-oriented person negotiating an open relationship with a monogamous partner. He seems to be avoiding some important compromises.
Two, he's not doing well in the dating realm. Rejection leads to insecurity, which leads to more rejection.
Perhaps he really lacks some basic ability to connect, to be sensitive to another person's moods, to listen, to be open. This is something I see in a variety of circumstances, from people who are just naturally very self-focused due to insecurity, to people with something more complicated going on like Aspergers.
To your original post, I was just going to say that he must be experiencing some insecurity due to the regular rejection he's been facing. As I read your follow up posts, though, it sounds like his insecurity is having a bigger impact on your relationship than just him moping around. I don't have any great advice on what to do with this information (or possible information), but think about how it impacts your connection with him.
And do keep in mind that he's facing more rejection than he's probably used to. However well his dating life went when he was a bit younger and single, he is going to have a harder time now that he's can only date women who are okay with him being poly. Smaller pool. And rejection after rejection is probably causing his ego to take a hit, and he may not know how to process that.
But if you're at the point where you're considering leaving, then you have a big decision to make, where you need to think about your own needs, and not about what it takes to comfort him.