What caused our yorkie puppy's hair on top of his head to thin out?

We saw him at 4 weeks of age when we put our deposit down on him. He was a picture of health and had shiny full hair. He was too young to take home then, so we picked him up from the breeder when he was 8 weeks old and we noticed a change in the hair on the top of his head. It is rather thin; you can see his scalp. Is this thinning of puppy hair normal? I am concerned for his health.Our 1 1/2 year old female has always had thick hair and never any thinning.