Did you see Coby's mom and dad? This is the best way to predict what your full-grown yorkie will look like. Some breeders don't breed to standard, and will breed any dog that they own, so not all yorkies have erect ears. Sometimes even though a breeder breeds standard yorkies, floppy ears occur, and while these yorkies shouldn't be bred, they make wonderful pets. I'm guessing since you had your dog since he was 9 weeks old, the breeder wasn't too informed about the breed, as most breeders insist that yorkies stay with mother a minimum of 12 weeks. There are several things you can do to encourage perky ears, but for a pet this really isn't important, and I think floppy ears are adorable, especially one up and one down!
