Yes, it's normal.
One of the very important socialization lessons puppies learn from their mom and littermates is bite inhibition - learning that biting too hard
hurts. Yorkie puppies should stay with their mothers until 12 weeks so he missed out on this very important lesson by being separated from his mother too soon.
You will have to teach him bite inhibition yourself now. Here's a good resource:
http://www.canismajor.com/dog/bite2.html