Poor kitty. Sounds like he has a lot going on.
It's really important he eats and drinks. If he stops completely that can be serious and a trip to the vet is definitely needed. In the worst case scenario, he can be put on a drip whilst other things are tried to get him eating and well again.
If you are giving him milk, see if he will drink special cat milk (most large supermarkets sell kitten milk and he could probably benefit from the extra nutrients from that). Cows milk is not good for any cat. It can upset their stomachs.
If he will only eat wet food, and pates, try making your own. Use tuna and other smelly fish as ingredients. Make one using chicken. Anything that can get some protein into his system.
Will regards to his depression, is it possible to give him a room to himself? With his own litter tray and feeding bowls, and bed? Right now it sounds as though he doesn't feel safe, and he's recovering from major surgery too. Keeping him away from your other cat completely will give him a chance to get his strength up and hopefully his confidence too.
Basically he needs a LOT of special TLC. I hope things get better for him and for you x