Didn't the vet give you some idea why kitty could be dehydrated with temp ? If it's an infection it could be resolved, if it's something different it might re-occur. Maybe pet's hospital could tell you that. I'm surprised the vet couldn't do more, to be honest.
What a decision to have to make. So sorry, hope it works out ok for him.
Humankind has not woven the web of life. We are but one thread within it. Whatever we do to the web, we do to ourselves. All things are bound together. All things connect.