[bjh]

Having raised yorkies for over 12 years and having owned a number of males I can tell you honestly that using a male for breeding can change their personality in certain ways. Each male is different so you don't know how they will react but most males when around a female in heat will whine, bark, quit eating and just act plain miserable but that is when the in heat female is kept in the same household. So to honestly answer your question, using him as a stud would not make him mean or aggressive, he would be the same sweet boy he is now but if you kept him in a situation where he was around a female in heat for 3 weeks and around other males at the same time, yes he could get aggressive toward other males. Also, males tend to mark more when around a female in heat.

Also you mentioned your male had a birth defect, no tail. While that is not a life threatening birth defect it would concern me that what other type of inheritable birth defects genes he might be carrying. There are many risks in breeding so since you stated you are not interested in becoming a breeder, I feel it would be best to not breed your male just to get a puppy but instead just go buy him a perfect little companion.