Why Is My Yorkies Hair Turning White?

Since he's a puppy, and is not geriatric, he is actually just growing into his primary coloration. Dogs are often different colors when they are puppies then they are when they reach adulthood. They pretty much grow in kind of the same manner that a baby does. You can have a daughter that is born with a large shock of black hair, then a few years later she's blond.
It's not a disorder. It's just part of the life cycle.

It could be natural colouring based on a breed mixed in somewhere down the line, especially since he is so young. I have seen a lot of yorkies that have some white or silvery streaks in the black part of their fur, and I think it's just adorable. Also if you have washed him with a whitening shampoo, it may make the white more noticeable. My terrier mix is red with white on his face and feet and the tip of his tail, but when I bathe him with a whitening shampoo, if you look closer you see white hairs all down his back too and they are always there, just more noticeable after the wash.