Shih Tzu Puppy Losing Hair

do puppies lose their puppy hair to grow thicker hair??
my sister has a 9 week old half shih tzu half yorkie. he has been losing a little hair lately on his belly and tail. he goes to the vet for regular check ups and everything so we know it isnt mange. do u know wat is wrong??
They will soon loose their baby fur. they dont have their soft, fluffy hair for long. They need to grow adult fur.
