Well, it's a little hard to tell from a picture, but I think he looks really good there
(they always look skinnier after a hair cut
). If he did loose some weight since then, he could probably stand to gain that little bit back (it is hard to tell from the pic, so trust your vet, not me lol), but don't go too crazy and end up in the opposite direction, lol.
It's honestly better to keep Yorkies (any dog for that matter) on the "trim" side anyway, it helps prevent joint issues. You didn't starve him, don't worry. It's good that you monitor their body condition so closely, too many people have really overweight pets and don't even realize it or take the proper steps to fix it. Alot of the time when starting a different food some adjustments have to be made with monitoring - the package amounts are just guidlines - the only way to really know is by how the dog does on that amount after a while, you're doing everything right