Sick Kitten?

I've taken my two month old kitten in to emergency vet today and I have absolutely no answers :(

He was severely lethargic, diarrhea (like two or three times in the last two weeks), constantly sleeping. He came back negative for panleuk (wet and dry) feline leukemia virus, ultrasound was clear, blood glucose was normal. They wanted us to hospitalize him overnight but we couldn't afford to. They told me they assumed it was some kind of viral that is making him feel sick.

We are monitoring him and he was given subq fluids but I want to know if anyone has any advice on how I can support him and help him get better. I don't want him to die :(