Didn't the breeder know or are you saying she said that he's a purebred? That's pretty big for an 18 week, but it's possible. Ears are longer than most Yorkies so it might be a breed with longer ears, but even that isn't always the case. Unless a breeder breeds for specific things, other traits will pop up and a purebred, can look like another breed. You can do a DNA test,
Dog DNA Test, Dog Breed Test - Wisdom Panel Canine DNA Testing | Wisdom Panel if your curious, they are becoming more affordable. Whatever he is, he's adorable, and looks like he'll be tall!