My husband left me for another woman can I keep my son from being with him and his mistress/

You don't have any legal basis to keep your son from visiting his father in this scenario. I am sorry your husband has betrayed you & your marriage vows, but you can't keep your son away from him simply because he is with another woman.

Depends on what the custody agreement says. THe fact that he is with another woman will not be allowed to be entered into consideration since it has nothing to do with custody.

If the custody agreement gives him custody or visitation, deviating from it for whatever reason could end you up in a lot of legal trouble including jail.