Well, you see in the olden days it used to be finding porno mags in plastic bags in ditches, but now you don't even have to leave the house. Have you got some parental control locks on the puter? Do they exist?
Definitely get parental controls on your pc and either you or his dad need to have a word about the problems of looking up this sort of thing on the internet - unsettling images, abuse etc. Maybe this is your cue to have that conversation now?