Hi Concerned Mother,
1. Is the mother complying with the mutual agreement?
2. Is the mutual agreement sealed with a consent order? If not, and the mother's conduct is not in line with the mutual agreement, your son can try having the mutual agreement sealed with a consent order. The effect of this is that if the mother continues to breach the mutual agreement, she will not only be breaching a private agreement but also a court order with far more severe consequences. To do this, your son will need to apply for a consent order with the Family Court.
3. Otherwise, if the parents cannot agree between themselves on a child custody agreement, your son will need to apply to the Family Court for a court order. Again, this will be with the Family Court.