Make sure the rat is hot enough. Let it thaw in the fridge and then put it in hot water until its warm all the way through. Make sure ur using tongs so he doesn't get confused with the heat of ur hand or any body heat. Has he ever been exposed to high temps or any chemicals? Does he show any signs neurological issues outside of feeding time. When u have him out flip him upside down and see if he can right himself. But I think its just prey not being hot enough