welcome Lina sorry about the post edit mix up ….
sorry to hear your little fella isn’t feeling well.. hydration is very important so that’s when the vet have him what is called Sub-Q fluids under the skin.
I’d you feed him dry food I would stop, for now at least. if a cats is dehydrated dry food really inst a good option especially if he’s slow to drink on his own. cats that eat dry food need lots of water. if he’s on wet food then I would add water to his wet food that way when he eats a little he gets the water too.. I can’t highly recommend enough to get your hands on some goats milk ASAP it helps to settle an upset stomach and is pretty irresistible to cat so that might encourage him to drink on his own.. also very important!!… get some honey ASAP and if you feel his energy is still sipping rub some in his gums this should help to bring up his sugar levels and I cress his energy and might also encourage him to eat a little more.. if he still doesn’t appear to be improving I’d go back to the vet incase he needs more Sub-q fluids to help perk him up..