You can try to add some canned food with the dry food, i did that do Mojo as soon as i got him and it worked
Check the amount of food you are giving daily, make sure it is the amount recommended for his age and weight. Yorkies are so small and have such a small stomach that they might eat less than we think is normal..
Also, having a regular schedule of feeding time helps, i do not leave food out for more than 1 hour.