How old is he and how much does he weight, have you thought about hypoglycemia? Yorkies can use up their energy reserves rather quickly and one of the symptoms of hypoglycemia is lose of appetite. Many people recommend free feeding for this reason. You need to be familiar with the signs of hypoglycemia of he's under 4 pounds, or less then 6 months. They also go through periods where food isn't very important to them, and if you give too much variety at this point the will become extremely picky.
Toy Breed Hypoglycemia