Gosh. I wouldn't be furious with him. Unfortunately it's normal these days for a twelve year old to be starting to be curious and look for porn online. Hell, I'm thirty, and it was normal when I was twelve!
I'd just tell him you saw he'd been looking at porn and that those sites are supposed to be for adults. And ask if he has any questions or wants to talk about anything he's seen. Remind him that although these sites as clearly popular, they rarely show what normal sex is like between two loving partners, and that when he grows up he'll realise when he meets someone that it's a world apart from the stuff online.
I'd also be saying 'I know I can't stop you from looking at it, so although I'd rather you waited until you were older I hope you know you can talk to me or your dad if you see something you are worried by or is upsetting, it doesn't have to be a secret'
Because the truth is, you can't stop him. He's twelve, not two. He will see it and that's out of your hands, even if you block him at home he'll see it on a friend's phone or someone else's PC, or a magazine kicking around school. Or work around the parental controls if he's determined (the disrespect of that is a separate issue).
Far better not to imbue him with shame and guilt at this stage and make sure you model talking about it like it's any other topic so he feels he can talk to you if he needs to. Going nuclear will just make him feel embarrassed but it won't stop him, it might even make it more desirable and taboo!