I wish I had the perfect answer, but there likely isn't one.
My first thought was tell him birth name was a figment of his imagination and that person never existed.
Next thought was his knocking up your mother doesn't give him some sort of magical rights over your life. And I say that as a parent. I have always treated my own son as a gift, and an equal inasmuch as rules are concerned. Yes he wouldn't exist if not for that moment in time when me and my wife were actively seeking to have her get pregnant. But a sex act and a pregnancy does not imbue me with magical rights of domination. I had legal responsibilities over my son for 18 years yes, but I was not in any way able to say I had rights over him, I was just expected to keep him safe and teach him properly.
Perhaps that is why my son is the most accepting person in my life right now.
If any of my family were to rudely and defiantly deny me my preferences, I likely would be equally nasty back to them.
We only become victims, when we allow ourselves to be victims.