J.S.
You didn't mention... is this your biological son or your stepson? If it's your stepson, how's your relationship with him?
If it's your stepson, it might be a good idea to have Dad have a heart-to-heart with him to find out why he goes to these sites. Is he curious? Do his friends do it? And then explain why you feel it's inappropriate (it devalues sex, which is an act of love, it doesn't show respect to women, etc). At 15 he's plenty old enough to understand what is and isn't respectful, and plenty old enough to show that respect.
Good luck with this!