It's really hard to tell... he could be full yorkie, but usually they are fluffier than that. It's possible he could be a yorkie/chi cross. I do think he is definitely at least half yorkie. If you really, really want to know, there is a DNA test you can buy at PetCo. It's kind of expensive, but it could be worth a shot.
Canine Heritage XL Breed Test Available Online at PETCO.com