I think the most important thing you can teach a Yorkie is to come to you when you call and not to bolt/run when the door opens. I've seen this behavior with two of my dogs and it is so scary!
I'm terribly sorry about your first dog
Do you think(just out of curiousity) that he became territorial because he felt he was protecting your son?
I've got to be more open to training with my two girls. My husband is excellent with them and they mind him much better than they mind me. I'm more of a softie, but being a softie could be the death of them. Like I said, Daisy will bolt at times. We've been lucky with her so far, she hasn't run into the road, but I've got to get a handle on her.
Tony, my husband loves our girls, he just will not tolerate any bad behavior. Or should I say, he tries very hard to correct it.
I think you're taking the best approach with your Yorkie. I wish you all the best. Please let us know how it goes and share any tips you have.
Tammy