Dear Matt,I've known quite a few folks like this — and your description sounds especially accurate.The Flaky Friend is a tough situation. It's tough because, though you could easily push this person out of your life and be rid the nonsense … unless your mutual friends all do the same, you can find yourself being the one ostracized.
My advice: be cordial to this person, but not overly so. Don't do any favors (because you know they won't be returned). Never give a "him or me" ultimatum to your friends (if he's attending something and you want a drama-less night, simply decline the invitation).
And be firm in your insistence that he remain at arm's length (meaning don't join any road trips in which he's involved — even as a passenger he's guaranteed to scramble up everyone's plans).