I wouldn't change his food unless it's a low quality food, because yorkies tend to be picky. There are several reasons for him to not eat much:
1. yorkies are small and don't eat much anyway
2. he's in a new environment, and it may be a few days before he starts eating regularly.
3. they almost never eat what the bag says. I fretted over this for a long time, and finally found that she is healthy on 6TB of food a day, not 1/2 cup like the bag says.
If he's eating very, pick up a tube of Nutrical Puppy. They have it at petsmart, petco, and most other pet supply stores. Give him a dollop to lick off your finger a few times a day. It helps raise their blood sugar and it can stimulate his appetite.
Anyway, the reason I say don't change his food unless it's a low quality food is that yorkies can be picky. If you keep switching up his food, he'll always pause before eating, hoping to get something better each time, then he'll always be a fussy eater.
Chloe did this, (and still does once in a while). I put her on a feeding schedule, and even that took a few weeks to really kick in and work. I have also found that if I give her any special treats, like some diced cooked meats, I don't put it in her bowl with her food because then she won't eat her food the next time.
I'm sure you'll get lots of ideas; since I have a feeling that my yorkie is not the only fussy eater out there
