We have had a lot of members have great success with clicker training.
http://www.clickertraining.com/home/
Yorkies are headstrong and it takes a lot of work to try and get them to focus on you. I bet with the clicker training method you could work with him in your home for about 15 minutes a night. Basically you need to find some treat that he will go nuts over and everytime you want him to focus on you - you click the clicker (I think about $20.00 and you can order on-line) and reward him with a small treat. Soon he starts associating the click sound with a treat and will look to you for this treat. I know you are very busy but I just don't think it will work every other weekend during a walk.
And trust me I agree with you on this statement but unfortunately I am the one that gets them like this because of rescue and I believe it is a high fear issue which makes them go after other dogs not dominance: And I don't think it's O.K. to chalk it up to "typical yorkie behavior". Just because we have little dogs that are spoiled and well-loved doesn't mean they have to be little beasts that embarass whenever they're around other dogs. There's got to be some answer to his behavior in this regard.
Do a search on YT as some members have posted actual video's of their Yorkies using the clicker method.