Surgery isn't an easy thing for an animal to go through. It is obvious he knows something is different. Give him some time to heal and report ANYTHING odd to the vet. I’m sure they won’t mind your updates, including the lack of appetite. Sounds to me like it is still too soon for him to bounce back but it wouldn’t hurt to notify the vet. Just give them a ring and they’ll most likely put your mind at ease.
Poor kitty. Hope all is well.
I would say it's probably normal considering what he's been through. As long as he's not running a fever or has redness in the incision area he's probably going to be fine. Did the Vet want to see him again to remove the stitches? Or are they the dissolvable? Better keep those hair ties put away. :)