Errrrrrr hang on, I assume as he is your dad that he is old enough to be looking at these web pages so what gives you or anyone else the right to say what he can and can't do, yeah if he leaves pages open or makes desktop displays I think a word would be in order but leave the guy alone.
If the content it's against the law then report him other wise install
ccleaner to delete all traces.
To be fair though as you are no doubt not telling him about this thread I think if someone comes up with a solution to
YOUR problem then he should be allowed to ask how to get rid of the thing you put on..