what name should i use on facebook without my parents finding out?

[Magical Unicorn]

You could use your middle name instead of your last name. A lot of people do that, actually. And for your profile picture you could use one that isn't of you so that he won't be able to tell if it's you or not. And yeah I know obviously your dad knows your middle name but you'll still be harder to trace that way...Also, when setting up your facebook use an email that he doesn't know you have. On facebook you can find people from their email addresses, so if you use an email address he knows you have he could find you.
To really be untraceable, block him. That way he won't know you have a Facebook at all... I won't recommend this though because if he finds out you'd probably get in a lot of trouble.
Hope that helped.