I would have no guesses as to what the original problem mighty be (was he vaccinated recently?), but I do know that antibiotics can cause digestive upset in many kitties and result in them losing their appetite. I would make sure to give the Clavamox with some food, and if he's not eating enough, talk to the vet about discontinuing it.
At the very least, start giving him some probiotics to counter the annihilation of his gut flora.
Have you tried offering him plain chicken baby food? How about some lightly boiled chicken breast with the broth? The juice from a can of low sodium salmon will also help get some extra fluids in him.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler