Absent parent

I currently do not have primary care of my son. The mother is nowhere to be found. Whenever I am picking him up or dropping him off I only see the grandparents (the mother’s parents). My son is 5 and claims he hasn’t seen her in a while. The court order states that I should have current contact information and address of the mother. Currently I have niether. I am trying to file a motion so I can relocate due to a job transfer. I tried to file it today but the courts said I couldn’t without a current address for her to be served. I ask her parents what her address is and her current phone number and they keep telling me that they will call me back and I never hear from them until the next visitation exchange. I was wondering what my next course of action would be as to filing for vilation of court order.