What to do in this situation?

for the night you could use a LITTLE amount of milk, just watch for the "runs". in the morning, find a way to get some kitten replacement milk from the store. if possible for advice, call the animal hospital on how to help. call the vet in the morning, plus, if he will eat it, you can soak a small amount of cat kibble in warm water and canned food (if you have any) and make a kind of "mush" out of it.