Ear mites are contageous but are more of a cat thing than a dog. Chances are the breeder has cats????
Anyway, you were right to have a vet check him out. Just keep an eye on him. I know when I had a dehydrated pup, I force fed her pedialyte. They get down fast and once they are down, it takes a lot to get them back up.
Continue with the nutrical. You don't want the pup to get hypoclycemic. Also, maybe you could boil a chicken and shred the meat and feed that for the extra protein.
Best of luck and keep us updated on how it goes.