Change of parenting agreement

My wife and I are separated and she has a son. Her and his father have a mediated parenting arrangement that has been in place for years. My wife has since abandoned our relationship and our 2 kids and has agreed for me to have primary custody of our 2 children. I have been a father figure to my stepson for 8 years and I want what is best for him. His dad lives in TN and he pays child support and get him through the summer and long weekends throughout the school year. My wife is not in the best place to parent her son. She is abusing alcohol, RX drugs and sleeping around with different folks, not to mention she has no job and is living with a friend. She is trying to get a job working in a bar. My stepson’s father is stable and willing to get custody. How can he go about attempting to get custody and have the parenting arrangement changed?

He needs to file an action to modify custody in the state where the agreement was entered.