Bring it to a vet or reputable animal shelter. It may be ill.
It is hard to recommend food without knowing its age, and yes, it may be ill. Bless you for trying to help.
Call me crazy, but have you tried a can of cat food?
I'm currently handraising a kitten, I've probably done this a few dozen times now. Cow's milk can kill a kitten pretty quickly. If it's a young one especially, it needs a formula called KMR ( Kitten Replacement Milk). PetSmart, Petco, etc, even feed stores carry it. It isn't cheap. If the kitten isn't peeing and pooping, it's probably young enough to need an experienced hand. Genitals have to be simulated to get them to go, may still need a bottle- and aspiration is always a risk if you're not adept. A very is probably your best bet, these little guys go downhill very fast. Without KMR and elimination he probably won't last more than a day.