Are you in Florida? It's against the law to sell a puppy that young! Puppies must be eight weeks old under Florida law.
Florida also has some of the toughest puppy lemon laws in the country.
Florida Pet Lemon Law
I know it sounds hard hearted, but the smartest thing you can do is return this puppy, report the breeder and get the purchase price plus vet bills back. You could spend a fortune on him and he still could die. Even if he lives, he most likely will have socialization problems from being taken from his mother so young plus very likely genetic health issues.