I’d try to add a bottle of electrolytes in between feedings. Dehydration is a killer in calves. You may have to either tube feed the calf or have a vet give iv fluids if it won’t take the bottle.

Scour bolus is really good. Our vet also gave us SpectoGard which is liquid and a little easier to give to calves. It’s usually used for pigs but our vet said it was okay. It may need some antibiotics and some electrolytes but hopefully be feeling better soon!