No matter what, your son will face temptations, we all do as we go through this life in our sinful flesh. Is he saved? If so, he is likely dealing with a weakness to resist this certain temptation.
If he is saved i would bring up the fact that i found the content and tell him what the bible speaks about lusting. I would teach him that what he finds on the internet does not even compare to real life sex when you are married in love, and that those images/videos can really hurt a real marriage in that area, that those things are skewing his views of what he thinks it should be like once he becomes married one day.