In Pennsylvania, a judge has ultimate responsibility to determine the best interest of a child when parents cannot agree. The custody statute lists 16 factors (23 Pa.C.S. Section 5328) that a judge must consider when determining the child's best interest, although number 16 is "any other relevant factor," so the court is not limited in what it can consider. One of the factors is the "well-reasoned preference of the child, based on the child's maturity and judgment." A judge would consider your son's preference, his age, and his reasons for the request to live with you, along with the other factors in entering a custody order.