Puppies are mouth-focused for quite a while as pups - it's just the way they communicate and learn about their world right now. When the puppy mouths inappropriately, you should "squeal" just like his littermate would if he hurt the littermate -- that's a language the puppy understands. He will get less mouthy as he grows from this current stage.
He wouldn't be quite as mouthy had he stayed w/ his Momma until 12 weeks, which is the age yorkies *should* stay with their Momma before going to a new home. The Momma teaches them / socializes them during this crucial time. Because you have him too early, you will be required to act as his teacher as he knows nothing right now, he's just a clean little slate.
Btw, welcome to YT
!