Teenage son is seeing a christian girl, and has committed fornication with her. Son also prays sometime and fast during Rammadaan when hes in my presence. What can I do to get him in control.

My son is seeing a christian girl, and I have learned that he has fornicated with her. It happened at her prior guardian homes. She was place with her grandmother after the incident was discovered. My son hears no logic when it comes to this young lady. I've taking the position that if he insist on seeing her than he will pay for his own sins because he is responsible for his own deeds of action. From my knowledge he doesn't bring her to my home, he does sneak and call her when I go to bed at night. Please advised if/how this should be properly handled.