Ouch, poor kitty! Have you called your vet to tell them what's going on? Anesthesia can certainly take it's toll on an elderly cat and there is often a longer bounce-back, but I'd be concerned if your cat's appetite doesn't pick up soon. What do you normally feed him? Can you try offering him some plain chicken baby food (no onions) with a little warm water added?
"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