I'm no expert with this type of thing, so please just take what i say as advice, but i would say something along the lines of that you don't care that he did it, you just want to know the truth, and you don't want him to continue doing it.
Also how did you find these sites on his computer? I know a fair bit about computers and i can tell you with good accuracy if this is his fault, or they were put there by someone else, or a virus.
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