It's not uncommon to be a little lethargic and have a decreased appetite following a surgery. If he is not on any pain meds then I may consider treating with something like Buprenex in case it's pain that's making him not want to walk around or go to the food bowl to eat.
I'm not terribly concerned as long as he's eating something and keeping it down. If he were completely anorexic then I'd be more concerned and probably want to treat more aggressively. The inflammatory stages of healing are days 3-5 and that is the most painful period so I would guess that in 1-2 days from now he'll bounce back and start acting more like himself. The main goal is to make sure he's staying hydrated and continues to eat/ drink some. You can also try some canned food to increase his water intake and also get some more calories into him.
I hope this helps, let me know if you have any other questions.