Welcome to Yorkie Talk.
My boys will not accept treats from strangers, including people at pet stores. Probably a good thing.
I would not force treats into his mouth. For training, you can try using whatever he regularly eats. My main reward for my boys is pieces of their kibble.
He may not be food driven. Try toys for rewards. Or he could still be adjusting to his new home. Let him take things on his own terms.
My boys' favorite treats are steamed plain chicken breast, parmesan cheese, and a few other things. Very tiny quantities. Treats should never exceed 10 - 15% of total daily intake or you risk upsetting the nutritional balance a dog needs.
Definitely put an end to feeding from your plate, or table scraps. It will encourage begging (I have been a little naughty in this area recently). You also risk giving him something that could make him very sick.
Be sure to review the list of foods that are toxic for dogs, including grapes, onion and onion powder, raisins, chocolate. Here is a great infographic on toxic foods:
https://www.pinterest.com/pin/86553624062406721/