[LunarBerry]

Yeah, some pups will eat lots and some less. As long as there's no health issues like he's lethargic or doesn't seem well, then this is normal. I find most yorkies are good about eating their fill. If he's eating less, he's probably not as active as the other pup. Usually the ones that are more active eat more, as long as he's eating regularly and isn't severely under weight. The puppy formula will give him what he'll need and if it's a higher quality food, then they'd be eating a little less of it anyways because it's much more nutrient dense and higher calories.
Oh yea, pups tend to shrink and expand one week to another before starting to level out, lol My furbaby did that the first month or two, one week being kind of fat and then next week kind of skinny ^^;;