I can't even come up with a guess . . . but I'll try. Maybe he did find someone for awhile, and then it ended and then he found someone again (or the same person.)

Maybe, back when he first distanced himself, he thought you were getting a little serious and didn't want that. But it's odd that he would go away, come back and go away again.

It might be best to just withdraw completely yourself. My guess is that you're not the only woman he's doing on the side.