Have you tried FortiFlora? It's not the absolute best thing for them but it's definitely better than them not eating. My cat was 'diagnosed' with lymphoma (didn't do a biopsy so don't know for sure), but when her appetite gets a little too low for my liking, I just sprinkle FortiFlora on her food and she goes crazy for it. I just pick it up at the vet's office, and it's pretty inexpensive!