hey,
i'm not a parent, but i have to say i think the idea of talking to ur son and his friend together sounds good...it's not to unusual for teenage guys to look at porn (i've got an 18 y.o. brother, he went thru a phase a bit like that a couple of years ago, but my parents just ignored it, so it's good that ur planning to talk to ur son) but i can understand how u'd find it disturbing!