Poor Stash, poor you.

Its up to you if you want to keep him in the bathroom or not. Does he seem comfortable if you leave him out of the bathroom? I guess its just a matter of wanting to clean up the carpets.

Its good you got him in for the fluids, they get dehydrated so easily. Kitties get diarrhea for the same reasons we do. Maybe he ate something different?

Did the vet do any blood work on him?

I hope he feels better soon. You too. Its so upsetting when our furkids are ill.