Should I start to worry yet? Well I already am, but how worried should I be?

Background: Adult, probably male, I've had him since summer 09. I adopted him from some kids who weren't feeding him enough. He was always a very good eater for me. Always fed in his tank, small f/t rats.

He's in a 40 gallon breeder. Aspen bedding, two identical hides, water bowl, and three sides completely covered. Undertank heat pad on a rheostat, temps checked with a temperature gun, around 90-93 on the warm side and room temp (76-78) on the other. The front is partly covered with fake plants so he can feel more secure.

He started refusing food in January for no apparent reason (there had been no changes to his environment or anything like that). He often does for a few weeks in winter or spring, but this time he never stopped. He should be around 1900 grams but he's slowly dropped to 1500.

He has been checked for parasites and he's clean. No sign of mites. His mouth and nose are clear, no signs of illness, and he sheds without trouble. I handle him only about once a week or less when I need to clean his tank or check on him. He's active and curious when he's out.

He often 'acts' hungry, but doesn't strike the rats I offer. My other BP takes the rats he doesn't, so there's nothing wrong with the food at least. I've also tried weanling rats but I don't currently have access to other sizes.

So what should I try first? Just keep waiting? Larger rats, smaller? Is it dire enough I should try live or freshly killed or what?