I truly am not sure if he's a purebred or not. It seems that he would be, unless the breeders are trying to pull a fast one on you.
Maybe he will just take some time to get the long fluffy coat, some develop differently than others. I really don't have tons of yorkie experience since 2 of mine are mixes. But I'm attaching a picture of my yorkie girl at 11 weeks. Not sure if that will help at all or not.
I know you're worried and I so wish I could help more
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