Technically, yorkies are considered "adult" around 6 mths old, according to most breeders.
Many, many, MANY vets are very uneducated about food / nutrition in general, but most especially about raw. Don't let the scare tactics get to you
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Primal is AAFCO certified, which means it's balanced in the exact same way that kibble, canned, freeze dried, fresh etc is balanced. He is getting all that he needs with this food. It's not the amount of food that ensures getting nutrients -- it's more like it's the correct amount for body weight while AAFCO balanced at the same time.
Rest assured that your kiddo is getting one of the most premium diets on the planet!