Classic Fancy Feast is about the safest cat food, I have found, for my cat who has a reduced immune system. There are 3 fish flavors she tolerates quite well. Also, you might want to consider some probiotics since he has had antibiotics, to repopulate the good bacteria. There are cat versions, but if they are expensive you can see if you can find a human version with the same ingredients and use that instead. Are his bowel movements good, do you think he has trapped gas? If so, he would be in pain and won't eat much because of that. A few drops of mylicon (or non-name brand of simethicone) into his mouth would help him to release any trapped gas. Good luck!