Hi XXXXXXXXXXXX,
I'm really glad you have an appointment tomorrow - sounds like you are a very responsible cat parent!
Drooling can be caused by many different issues - some examples are dental disease (even in a young kitty), nausea, and even a liver problem called a "shunt."
If you're cat is otherwise acting normally - normal activity level, appetite, urinating and defecating ok - it is very likely he will be fine until tomorrow. If you are seeing any other symptoms besides the drooling, let me know...
Sincerely,ERDogDoc
Cat Veterinarian
Small Animal Emergency and Critical Care Veterinarian