He may be paranoid that you will "hack" his Facebook and has no idea how security works.

You could get him to use a separate browser, or profile (Chrome handles separate user profiles really well and you can have separate short-cuts for each profile.)

You could log all network/browser traffic, (don't bother with key-loggers as it'll only log key strokes and not show what webpages he visited)

Or you could just do what I did and change the wallpaper to a big MS paint message. I chose "Hide your porn better!" along with some top pics from the folder.
You could try something a little more subtle like "Stop clearing my browser history"