So, the first step is mediation, and it's good to hear that is under way, but it can be a time-consuming process unless your son wants to foot the funds to get it done faster through a private provider.
Realistically, however, mediation won't guarantee that he sees his children. It's a forum for the parents to attempt to reach agreement about the kids' care arrangements, but if no agreement can be reached, then either parent remains at liberty to do as they please (including withholding the children).
If, however, mediation fails and no agreement can be reached, your son will receive a s 60I certificate, which will enable him to file an initiating application for parenting orders with the Court. The application requires a minute of the parenting orders he would be seeking (such as when he sees the children, etc.), and an affidavit of evidence to show why the orders sought are in the best interests of the children.
What is the current situation with the DVO? Has there been a mention or hearing for it? Did he accept it without admissions, or contest it?