Was there anything, and I mean anything at all, different about the rats you've been giving him and the ones he's being offered now (assuming nothing else in his environment like substrate, cage furniture, room he's in etc has changed)? One of my snakes WILL NOT eat rats that are white, he acts like he can't see them. I know another that will only eat female rats. Any abrupt change in size can throw them off too, such as moving from pups to smalls. And sometimes they're just subborn jerks, and you have to out-stubborn them.