That's pretty major surgery so it's probably not surprising that he has no appetite. I think your instinct is correct, though--if he hasn't eaten by tomorrow, time to call the vet for some advice.
Did they send the mass out for a pathology report?
"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."