[Maximo]

For the safety of your pup, he should not be outside without some means to contain him in the yard -- a fence or a leash.

Training to stay within your property boundaries or to respond to recall would take lots of time and effort (the latter should be worked on by every dog owner for safety reasons). However, no matter how well trained, there will probably be that one time that training fails and your pup will run free. Yorkies are too small and vulnerable to risk having them run into the street or far from home.