My daughter is 15 now. The words do not exist for me to be able to explain to you the amount of damage that he has already done to your son, and the amount of damage he will continue to do if you are unable to protect your son from him.
I did not protect my daughter from my wife and those comments and behaviors, and now I'm reaping what I sowed (cutting, drinking, smoking pot, bad grades, bad boyfriends, bad friends, and early sexual activity). I'm not saying this will happen with your son, but I am saying it is not uncommon in family dynamics like yours.
It is a form of child abuse, and nobody will ever be able to convince me otherwise.
Take care,
Cyranoak