Awww, hope he's back to 100% soon.
I don't have experience of having a castration done (Peter was done at rescue), but I had Poppy spayed & I know spay is a bigger operation but she took a good 3 days to really start showing an interest in food so although it is hard try not to worry too much. The trauma of being away from home will have upset him, let-alone the loss if his little plums. Also his Mummy keeps squirting stuff in his mouth (obviously the right thing to do), but he's probably just a tad stressed out. I am sure he will be more like his normal self soon. At least he's washing etc, Poppy literally sat hunched for 48 hours, it was awful.