I'm assuming that he didn't tell you that the boot was cracking when he gave you the car back? He probably isn't the cause of the split boot, but if he didn't notice that it was cracking while he was under there, then that's just irresponsible of him. There is also the possibility that a stick, rock, or other debris may have torn the boot in that time. If you can tell that the boot is cracking in other places, then he will probably work something out with you. But if there are no other signs of wear, then you are out of luck, and he has no moral obligation to cut you any slack.