Is the porn being filmed? Getting your hands on it and presenting to judge would be the fastest most effective way.
While your son may not understand the reality of what is happening he is still affected by the energy of being forgotten and left so watch out for signs of it. Be prepared to support him in healthy ways that uplift and encourage him as a child and person.
One of the most important things is to NEVER bad mouth the mother to him no matter how much you want to. Love him but don't overindulge him. Find ways to build his confidence and feelings of being loved. There are many resources online if you google "helping my child feel supported through divorce"