Down this list are some links that may help you. Janet Jackson says that puppies that are not socialized with people by 12 weeks of age will be timid later in life. She also says that starting at 14 weeks (when the juvenile period starts) the puppy needs to be introduced to stangers (adults, children, and other dogs). However, I would stick with dogs his size and save the big one's until he's comfortable with the smaller one's first.
http://www.dogchannel.com/puppies/social/default.aspx
Do a google search for how to socialize your yorkie puppy and you'll find lots of info. Also, there are lots of yorkie books available with good info. I have a number of them.
Also, when a puppy acts fearful, the absolute worst thing you can do is to pick it up and comfort it. You are reinforcing their belief that they really have something to be afraid of. Just pat him on the head, instead, and tell him it's ok and go on...