Hi
I know the feeling well, something that might work, an old trick, Green beans and Fiber One Cereal mixed with a little bit of whatever regular food he likes to eat. The Cereal and the green beans full them up and they usually like it. Once you get him back in weight, be very careful to measure his food to keep him in weight. Always give veggies as treats

Also have you had his thyroid tested? Yorkies can have a problem with that
GOOD LUCK
Sparkling Blu Yorkies