Sorry to hear about your situation.
I was told at 8 that the man I called Dad was not in fact my bio dad. I found out as I came across my birth cert and it said another man's name.
If I hadn't have found that I don't know when I'd have been told or if I ever would have.
My bio dad was and is a utter cunt. It obviously isn't the same situation as you but I wish I never found out. I have nothing to do with my bio dad now.
The man who brought me up is my dad. He legally adopted me when I was 10. I have 2 birth certs. One with bio dads name and the other with my dads name.
Your son does deserve to know as its his life but at the same time I understand that him finding out he may be the result of something horrific could have an affect on him.
I think you do need to find out who his bio dad is though.
I have been raped but nothing like your situation.
If I was you I wouldn't want to tell him either.