My son's wedding has already been held. What's the point of my ex-husband asking for another wedding for his son?

Because your son probably didn't ask to be present when he got married, as the child's biological father, his son's marriage is a big deal, and as a father, he should be present, so it is understandable that your ex-husband asked for another wedding for his son. He just wants to experience the day when his children get married and witness the moment when they grow up.