What kind of sickness did my cat have?

Maybe he ate something or inhaled something, perhaps laundry detergent. This could have caused the upper respiratory infection and the drooling, it could have caused damage to his mouth and so on. IV fluids wouldn't go through his stomach, they would have gone through his bloodstream into his kidneys, but only after he was re-hydrated. The only way to really know what caused his death would have been to have had an autopsy done. Vets will do this, if they want to know or if the owner asks and can afford it.