Thanks everyone for your replies!
Bunker is my first yorkie--and first dog. When I first got him, I posted a thread to get advice because he wasn't eating anything. (Thanks for everyone who helped on that one, too!) You all said he was probably homesick and had a loss of appetite. Plus, he had another dog playmate, and Bunker may be grieving because of their separation. He's doing much better now, and eating a whole lot more.
I will be sure to take Bunker to the vet soon, just to reassure myself he's perfectly healthy--and just a cute lazy dog!