The vet couldn't say for sure, but it would be either not a pure breed puppy, or a poorly bred one. He couldn't say it's not a puebreed without conducting tests, but he's been around for a long while and has seen many yorky puppies. And Mojo doesn't look like one. He looks like a mix.
The litter is CKC registered, but he was the last puppy and we didn't see the rest of the litter. We were also told by the breeders that Mojo weights much less than he really does — we checked with the vet today.
So yeah... It's a good lesson for me, but it also makes me very sad, for Mojo and for breeders who just say things to make you buy the dog. But it's also my fault — I didn't go to a reputable breeder, but somebody who claimed to sell a "purebreed" puppy. And I need to recognize that and learn from it.