I would say, delete the files, clear his cache and his history, wipe everything. See if he says anything, and check back to see if they mysteriously return (BTW, depending on the OS of the computer, "temp" doesnt hold the internet cache, it is where install files dump things to use "temporarily" during set up, and programs like winzip stick things for short term use, which makes it more suspect if that is where they are).
If they return, have a chat with him.
For the record, while beastiality is immoral as hell and illegal an act, i dont know the legalities around the porn, i would imagine that changes state to state. Child porn however, can get you a meeting with the FBI, and that gets hard time where people into that dont last long, so i would give this a try sooner rather than later.
If he gets caught with it, nothing short of physically destroying his hard drive will erase the evidence, so its time for an intervention.
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