Is it possible to serve someone when you don't have a physical address nor know where they work?

My son's graduating in June. My child support plan says if I want support to continue through college years I must ask the court to amend the plan. Our parenting plan says if a parent moves they are to give the other the new address but, my son's dad only gave me a PO Box. I don't know where he works as its changed over the years and support enforcement isn't allowed to give such information. I can't serve him during visitation because he doesn't exercise his right to it. In order to ask the court to change our child support plan is it possible to still serve someone under these circumstances?