Miserable Child!!

I was divorced almost 4yrs ago. We have joint custody of our now 13yr old son. We are both engaged to be married this summer. My ex's fiance is a woman who has never had  children and has no clue how to relate to them either. She isn't fond of our son and he doesn't like her. When it's his day to go to his dad's house, he actually starts to cry, and gets headaches. The last time he was there, so many things happened that were unpleasant it was sad. He didn't want to go to dinner with them so he went to his room and locked the door. His dad threatened to take the door off the hinges! He panicked so badly he had an asthma attack and used up his inhaler. His dad gave him a coke instead of getting him a new inhaler! Then the adults went out for the evening and turned off their cell phones! The fiance told our son, "you came to ruin my weekend"....WHAT??? Who says that to a kid??
What can I do to get full custody of our son? He sees a therapist for depression. It is situational. When he is with my fiance and myself, he is full of life, there is a twinkle in his eye and he won't stop talking...LOL.
My ex has threatened take full custody of him once he is married, but I know he would be sick and so sad, especially since the future step mom doesn't like him. They also live half and hour from the school he wants to go to.
What can I do? He will be 14 in about 9 days. We live in WI. Is there any law that is on my side or my childs side that says he can have a say where he wants to live?
The therapist will not take sides. She believes he has the ability to find the tools needed to stand up for himself in the house and walk away from bad situations instead of pushing each others buttons.