Man, it is a good situation to start talking to your father about your reboot, shut down the filter and let him enter the sites, after this you will see what kind of porn was watched, you will tell your father that while checking your history you found porn sites, then you will easily catch him, you can catch someone when he is lieing. Besides you will see what kind of porn was accesed, and you'll figure it out who did it from your familly. Until it's not late save the situation, if it is your dad, it will be harder for him to reboot being in a relation with your mom:
1. He would have to tell your mom about the reboot, wich is not so easy to explain you were watching naked women and so on.
2. For faster results and better, he would have to be cold turkey for this period.
You were already in this situation, don't let others be in this shit, especialy your familly.
Cheers bro, I hope I was not hard.