Hi Alison! Welcome to Yorkie talk
it's good to keep a close eye on our little buddies as they can go downhill fast. There are many a Yorkie who have a finicky appetite but make sure your pup continues to eliminate (poop and pee) appropriately and especially if you have a tiny keep an eye out for any signs of hypoglycemia. Maybe your buddy is just going through a growth spurt and needs an extra nap? But if you are worried keep an eye out for additional signs of poor condition: any trouble waking or balancing, vomiting etc and like ann said, if you are still worried take him in for a check up
good luck with your pup!