Just curious, does he usually show up? Or usually not make it?
His reply is the equivalent of "I'll be there unless something better comes along".
I agree with just not inviting him anymore but I also recognize that there may be reasons why you don't want to do that. You can try to press him for a more definitive reply, but my guess is that if this is his MO, it probably won't work. Also, even if you push him and he then commits, there's the possibility that he'll still come or not according to his whim or other opportunities which pop up. And the possibility that he'll cancel at the last minute, or worse, just not bother to show up. Or even worse, say no, and then show up anyway!
All that to say, probably, there's not a lot you can do to change this guy's behavior. You can either quit inviting him, or just plan around his rudeness.