Can't Bury the Past

I apologize for this post being lengthy. This is difficult for me to talk about. After a 13 year marriage and 1 daughter my ex and I got divorced. I met my 2nd husband and after dating for 2 years we were married and then my son was born. I soon found out the man I married was an evil man. I divorced him and he went to prison and is now a registered sexual predator. Years later I ended up meeting an older policeman and we were married. He adopted my son when he was 4 and we have a great marriage. We never told our son that his dad adopted him. He is now 10 and we are not sure how to tell him.