I am a survivor

I don't know if I even belong here.

My advise is simple. Tell him the truth.

"You were having a nightmare, and I heard you say... So I think that means that you are a survivor of child sexual abuse. I love you, I am here to help you heal, is there any thing you want me to do?

Let him know what you know, and let him identify what role if any he wants you to play. If he doesn't want to deal with it, you can't make him.