Paternity was established at birth, there is a custody and support order, the child is now three, and the child still shares your surname. An AOP establishing paternity for the child will normally be treated as an agreement about the child's surname.
Depending on the judge, your ex- may have a shot at arguing for a hyphenated surname, and if you agreed then the only thing for the judge to decide would be the order of the names; but given that background it would appear unlikely that a court would find that it was now in the child's best interest to carry only his father's surname.

