Family

My son shares joint custody of his 4 years old daughter with his ex-wife having residential custody. In his property settlement agreement, he has visitation every Tue and Friday into Saturday. However, this Friday, he received a phone call from his ex stating that she would not be allowing him to see his daughter, and because she has residential custody, she is in charge of making the rules. What recourse is available to him other than calling the police to enforce the custody?