Also possible he has seen some inappropriate movie or video , maybe check out what he watches ...
is it possible that he is being sexually harmed in any way by someone and he is scared to tell anyone. i know from my own experiance as a person melested as a child that it is hard to tell anyone out of fear of the person harming him or un acceptance
What relationshipd does he have is his Dad around has he siblings, would anyone have put thoughts into his head, it sounds strange that he would come up with thoughts like this out of the blue, have you asked him Why ...