I make these sorts of social blunder's all the time, so maybe I'm not the right person to ask. But if it were me and my friend, I would say something like, "I totally understand that you're really busy but still want to hang out. Let me know when you're free to do something."
If the issue is that he is RSVPing for stuff he doesn't do, I would say something like, "I know that things happen sometimes. Next time though, if you think you might not be able to come, would you please give me a heads up?"
Not sure that I'm saying the perfect thing, but I figure that it can't be much worse than ignoring the situation and not having a friendship.