Hi, I assume there has only been a verbal agreement so far... relating to custody, and parenting decisions, and holidays, etc.
Have your son and his ex de facto/wife officially divorced (if applicable), and reached a binding property settlement? If unsure, please say so.. as very specific requirement must be met for a binding agreement to be reached.. and it MUST involve lawyers, and/or the Courts.. and it will not be binding if it is only a verbal or written agreement between the parties themselves.
Here is the prescribed Court reading for property settlements:
Next, if they do not agree wrt custody, parenting decisions... and/or overseas trips..etc. they MUST attempt mediation with a court approved mediator, and obtain a certificate of attempted mediation before they can apply to the Court for custody, and/or parenting orders.
Here is a register of mediators your son can approach to attempt to organise a mediation.
He should also read the following prescribed reading issued by the Court:
If he needs to take the matter to Court to seek the Court's approval to take his daughter overseas, please let me know and I will point you to the process to follow...
Please accept and rate my response.