If the vet can't see him soon, is there another office in the area with an opening? A back-up vet?
Panting aside, excessive drinking needs to be looked into. Ellie just went through some excessive drinking and it was a UTI.
This site has a list of possibilities:
Polydipsia and Polyuria in Dogs
If you end up waiting a day before going to the vet, it would be good to measure intake in ounces.