I think it would make for a great news story. Alert the media that your JW ex-husband is planning to violate the court order for access to your son. Get the police involved. Stand outside the stadium where the convention is taking place and picket with a sign that says
"MY SON HAS BEEN KIDNAPPED AND IS BEING HELD IN THIS CONVENTION IN VIOLATION OF A CUSTODY ORDER"
"JEHOVAHS WITNESSES AID AND ABET ABDUCTION BY NON CUSTODIAL PARENT"