Feral kitten found, need advice?

I found a very dehydrated skinny little kitten yesterday in the parking lot and promptly took him to the vet where he was de-wormed and given fluids. She told me to just have him rest, but he is just very lethargic and doesn't have much of an appetite. He is skin and bones, and had diarrhea this morning (including roundworms). Anyone have any insight as to if his inactivity is just due to being so malnourished? He's been drinking water and some KRM, he is very sweet and loving.