I have an adopted daughter. Unless you have always been close to them, I would not involve myself with your sons birthparents at all. Its up to your son to tell them anything and what he says, true or false, is between them. It is very common these days, whether we approve or not, for adopted kids to be contacted or contact birthfamily via FB. It is not always respectful or on our timetable.
My daughter was adopted from abroad and is trying to find her birthmother so far with no luck. If she ever does find her, I am sure she will tell her birthmother all about how abusive we are. I would not want to engage with her. I adopted my daughter, not her, and I don't want to know her.
You don't owe his birthparents anything. That is at least how I feel. I would not converse with either of them. Your son can do it. He is an adult.
