Slipnot are satanic, and by listening to stuff like that, you are letting demons have power over you. Sorry, but it's true!!
If I bought books on satanism, I would be letting demons have power over me, until I got rid of the books. Having anything associated with the devil in your house is bad because the demons that are attracted to them can affect the owner. Call me wacky, but I have seen some strange things happen and demons are very real. I had a poltergeist in my flat while growing up, but since converting and binning all the stuff that si supposed to eb bad, I haven't heard a thing.
Back on topic, if the pastor's son was listening to it he can't be doing a very good job of teaching his kid if the kid wanted to listen to Slipnot. If he doesn't listen to it, then it has to be because he himself knows why it is wrong, not because his dad tells him it is wrong. Such a harsh punishment IMO, will only make him want to listen to it more.
What would I have done? Sat him down and explained. If the kid wasn't a Christian.. leave him to it.