Sometimes one of mine has problems finding the head/where to start eating, if the rat is a little too big... And then gives up after 20 minutes, and I have to try again. :s But if I offer him a small rat, he seems to have an easier time eating it. So you can try with a smaller rat, one that he can even eat backwards if he chooses to, and see if that helps?

If he's not becoming very skinny though, I don't think you need to sweat it!