Due to psychological issues, can I get 51% custody rights of my teenage son?

I want to have joint custody rights of my 13 year old son with my wife. She has a history of psychological issues and I need to make sure that if required I can break a deadlock if it is not good for my son. Is it possible for one person to have 51% and the other 49% of joint custody rights? Is there an age where my son can choose who to live with? I do not want an outside arbitrator.