It could be that he's lethargic because it's winter. They can tell as the weather changes, and a lot of them will slow down a bit and not be as active. If he's more active than normal in his tub, I would double check the temps and make sure everything is perfect.

How much does he weigh? They do get lazier as they get bigger. And if the ambient temps in your house are a bit cooler than normal, it could just be that he's not willing to move from his warm comfy spot

As for switching from mice to rats, it may take a while, as I'm sure you've heard. I try to get mine on f/t rats right away, so I don't have to switch prey types multiple times (from live to f/t, from mice to rats, etc.). Try again in another week, and make sure it's good and hot. As long as he's not losing a significant amount of weight, it won't harm him to skip a few meals; when he's hungrier, he'll be more likely to eat. And as you probably know, as they get bigger, they tend to refuse meals more often. It could be partly winter/fasting related, and not just that it was a different type of prey.