procedure for seeking modification to child custody order under NY family law

Child custody/visitation.: I have primary placement of our son. My ex has visitation. We both have joint legal for medical and school records along with making major decisions. Our son is visiting this summer and is due to return home 8/23/08. On 8/17/08 my ex callled to inform me she was not returning the child. I requested that she put the child on the plane to return home as it is a court order. She refused. The ticket was at my expense. She has it in her possession. She has told me she is enrolling him in school where she lives and not to fight her in court as she will do whatever it takes to keep our son. What do I do? - Is this your question? Add additional information