It's probably an impulse control problem more than anything. Dogs and kids and a lot of us adults don't have much of it sometimes! I would start with the Nothing In Life Is Free method of dealing with him by just Googling the term and reading from the sites about how to start your dog looking to you as his leader and establishing you as a pack leader in his eyes. This buys you respect in his estimation, someone to be looked up to, obeyed.
Start some just basic obedience to help him learn to do what you say. In the Library Section of YorkieTalk are some basic obedience lessons to just teach the basics of sit, stay, lie down, etc., and to get him learning that doing what you say reaps an instant food or toy reward and praise from his leader. Below is the link to that Page. Go down to Guide Part V.
http://www.yorkietalk.com/forums/pup...w-parents.html
Once he's got the basics of obedience down and is respecting you as a pack leader, teach him "Leave It!". That is the thing that you can now start to teach him to stop what he is doing and when he does, he is instantly rewarded. That can apply to his chewing on shoes, going after a dropped pill of yours or - biting, nipping at feet and ankles. I will post a technique I have used for the "Leave It!" in a separate post.
That is how I would approach the problem. Another thing to try might be stopping all play and activity and stand still, totalling ignoring the dog with folded arms and a complete cessation of attention as soon as the dog starts to nip but I've found that if the dog has poor impulse control, he will just keep nipping as you stand there, though if you keep it up long enough, he will eventually get bored and stop the nipping. And then when you start to walk again, he starts over. You stop again, fold the arms, ignore. In time, that technique can work though it is a slow way to do it for some dogs. I will post the "Leave It!" teaching technique that works fastest for me in a bit.
P. S. Be sure and keep the obedience and all training fun and positive for the doggie! Good luck.