How do I request a modification of child custody?

I have had sole custody of my son for all 12 years of his life. As of June 1st he went to live with his dad( volunteer but court ordered) My son is extreamly unhappy. He has had to take care of younger siblings, complains of his dad alway saying bad things about me. wears riped clothes and shoes with no sole. Things I have given him ( books, gum, bible, glasses) have been taken away and hidden. He has lost almost 10 lbs in less than 3 months. Has seen porn while being baby sat by an uncle. etc. People (from church, scouts, etc) have seen him with his dad and say he looks extremly sad. I have recorded phone calls where his dad walks him what to say, and only allows him to speak to me 2 or 3 minutes. He is terrified to speak to me infront of his dad and recently told me he couldnt get in the car with me or he would get a whipping from his dad. I would like to know how to regain custody.