Hi and welcome to YT! If I'm doing the math correctly, then you got your baby when he was 9 wks old? Generally, Yorkies are not placed with the new families until they are 12 wks old. Not biting is a social action that is learned while with the mother and litter mates during those later weeks, but your little guy didn't get that. So that is a reason
why he may be biting....
....Now to stop him from biting, you've been given some good advice already...the best of which was be consistant with what you choose to do. Make sure that he has plenty of toys to chew on that he likes. A firm 'no' followed by a time out away from the rest of the family is what I would suggest.
In any case, good luck and welcome to the YT family.
