What do I need to do to get custody of my brother?

I'm about to be 20 years old and am engaged. My younger brother is fourteen and his parents are divorced. My brother has been living with me for four months now with no legal arrangements made. Right now our mother has custody of him, but her and her current husband are going through a separation (which I support) and before the separation it was not a great place for a fourteen year old boy. My brother is about to go the the 8th grade when he should be in 10th grade next year. Since he has lived with us his grades have improved enormously! I'm very proud of him. But my fianc�e just got a job that is about five miles from home and we are going to have to move there. Our mother and his father have agreed to let me take him but now I have no idea on how or who to talk to. I keep thinking just the attorney general but not sure. Is there anyway that we don't have to get a lawyer or go to court? Both of his parents have agreed and his father will switch the child support over, just don't know how. Any advice would be great, thanks.