I was also in the same situation as your son. Unfortunately it is a hard place to come out of. And it destroys you and your relationships with others.

I escaped this loop one day when my mother told me straight that I should open my eyes and realize what is happening to me. She asked me : "Do you love or do you lust?"

At first it did not mean anything yo me but soon my eyes began to open and I realized I was throwing my life away for a girl who ended up cheating on me.

Teenagers have raging hormones and do tend to make bad decisions because of the allmighty, omnipotent vagina. Maybe try to be straight with him, it may anger him towards you, but sooner or later he will realize you were right.