Blackmail.
Yes, blackmail, that's your answer. Hop on your pop's PC when he's at work tomorrow and find his porn. Upon aquiring said pornogrphic directory information, write it down. Make sure to make a copy of the folder(s) should he attempt to delete it.
From there on out, you can be creative. Though I would suggest threatening him with the paper with such comments as, "What, isn't mom good enough for you?" when you mother is nearby and can hear his following outburst. OR simply just leave the paper where your mom can find it, and watch the fun that is guaranteed to ensue.