I'm sorry to hear that you are headed for divorce, but am glad you've taken the action you need to be in a healthy place. It's tough, because you can't have a conversation with your son telling him that things are not true -- that just makes you seem defensive. The best thing to do is the opposite of what your husband is saying. For example, stay informed about his schoolwork and be ready to help him.
Tell us about your other two children. How are things going with them?
RC