I'm glad you are taking such good care of your little puppy. You have gotten some great advice here. I'm pretty sure that pet stores who sell live animals have to be USDA registered and I know a breeder who sells to a pet store has to be USDA registered. They USDA has rules that any puppy sold and/or transported has to be at least 8 weeks old. If you are sure the puppy is younger than 8 weeks old (does he have papers?) please contact USDA and your state agriculture department to report the pet store and breeder. No one should sell any Yorkie puppy that young.
Good luck with your new baby.
Teresa