6 weeks old was pretty young to bring him home. A responsible breeder will keep their pups until at least 8 weeks old. As to his tail being so short, it kind of goes hand in hand with the type of a breeder that gets rid of pups at 6 weeks old :(

No, he won't grow into it. That is his tail. While a show breeder may dock longer than is typical, you still need to have a tail with a few vertebre at least. Pet breeders tend to either cut the tails off themselves or simply tell the vet to remove it without giving any other instructions.