You could try Hill's Prescription diet I/D. I worked at both an emergency and primary care veterinary hospital and with all the dogs that came in sick, not many would resist this food. You have to get it as a prescription through your veterinary office but if you ask your vet i doubt they would refuse to let you try it. However, my dog sometimes goes a whole 2 days without eating occasionally but she is 50 lbs.

Is he active? Whenever my dog stops eating it's because my routine walking stops (like when I broke my finger). Maybe you might want to try routine exercise to raise his metabolism so he gets hungrier more often? Worth a try! My dog seems to have more appetite if we get more exercise.