I think blond_law has it spot on.
Why does he keep coming back? Because he can! Giving a man commitment-free sex is like giving buns to an elephant, he will stand there waiting for as long as it takes to get some more.
If you are happy - fine, as long as you view it on the same terms. If you have thoughts of him coming back, forget it, he would have done so if that was his intention.
If you are enjoying commitment free sex with no strings, then fine, but I suspect you think if you sleep with him often enough, he will come back.
Sorry - not going to happen. Find yourself a new man and look forward, instead of wallowing in comfortable nostalgia for a relationship that is gone, and shows no signs of coming back.