You must get this under control because you don't want to be responsible for him biting someone.
I suggest you start with obedience training; or perhaps seek out a private trainer. Something you can try at home is NILF - Nothing in Life for Free. This is a very gentle way of letting your yorkie know that you are the leader. They must sit before feeding, treats, before going out the door . . it could be a good place to start.
Nothing in life is free - NILF - dog training - Article on Pets.ca | Pets.ca
Keep your dog active and exercised; walk him daily and get him used to being around other people.
I'm sure others will chime in.
Best of Luck.