He has options on preventing a government shut-down, but I understand why he doesn't like any of them. He does not seem the person who quite wants to confront the crazies of his party even if it is the most obvious thing to do.
It's possible that he feels a kind of debt to the people who got him into office for all these years. But that, too, could eventually bite him in the ass. Somewhat like a flea turning on the dog he rode on in, you might say.