Ok here are somethings that has worked for me many times.

1: Offer a live mouse. I've had animals that wouldn't touch rats nail mice and then switch back to rats

2: cover the tub/tank for a week or so so that no light gets in then offer him a mouse late at night.

3: Take him for a car ride. I know its sounds weird but there's something about the vibration that will spur feeding in some animals.

4: The smaller tub is a good one to try with a P/K item