Was he vaccinated at the same time as the neuter?
What foods have you tried giving him (ie anything like tuna or salmon, stinky canned food such as Fancy Feast, etc)? Will he eat treats like Temptations?
What antibiotics did the vet prescribe? Unfortunately many antibiotics will cause digestive upset in cats, so I'm not sure giving them on an empty stomach is the best idea if he's already inappetant.
"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