child stealing ?

My sister is divorced and her ex has full parental custody of their son, Justin. They alternate visiting weekends. This Thanksgiving the ex picked up and moved to AZ from CA without notice. My sister has been calling her son's cell phone and he said his dad said they are not coming back to CA. The ex will not talk to my sister to give her a home phone number or home address. My sister is trying to talk to him to bring Justin back to visit this Christmas. She called the police and they said since the child is not in apparent danger they do not pursue these cases anymore. It seems like he does not want to talk to my sister anymore nor does he feel that he has to allow my sister visitation.
Is this child stealing?

Is there a court order on her visitation? What does it say if so?

He cannot violate that order or he can be held in contempt...

The court order says Alternating Weekends. If he's guilty of Contempt of Court, could he lose custody?

Yes he could, or be fined, jailed, etc.