Christian Parenting on the Edge

My 17 year old son has been very secretive with me lately, recently he has started to refuse to go to church with the family and tonight when I was going through his room I found a magazine with naked men in it. He obviously has a girlfriend that he is hiding from me that brought that magazine into my home and I am afraid they are having intercourse and I am greatly concerned that he is going to get her pregnant.

What should I do about this?