Hi,
Just my 2p worth, but....
I don't personally think you should be getting that bothered about this and perhaps more imoprtantly, need to be careful how you deal with it (if you haven't already).
When I was about 13 / 14 my mum found my stash of porn. She made me go out into the garden and burn it. I now look at porn on the interweb and am not ashamed of the fact. She didn't stop me, or even reduce my interest in it. All that really happened is that I learned to not get caught and it took me longer to build a decent collection.
So I have been looking at porn of various kinds for the best part of 30 years, I am in a stable relationship, we are engaged and I have never treated anyone with any less respect than they deserve. I like and respect people in general and women especially as I was brought up in a very matriarchal family.
also remember that this has been going on forever, ever since Adam caught a sneaky look at Eve putting on her figleaves!
The main thing is that he is brought up in an environment that encourages respect for others in all aspects of life.
As has been said before, looking at "normal porn" on the web is nothing special, but again if it is anything too dodgy, then just have an adult chat with him. At 12 years old he knows more than you think, and probably a lot more than you would like to think he does.
I just believe that you won't stop it and so maybe the best approach is to make sure he understands what he is doing and why.
Don't take the easy option of fooling yourself into thinking this will stop, he'll just learn better how to hide it from you, and I'm sure there are plenty of mates at school who can tell him how or have PCs that are not monitored by their parents. Do you want him to do this in someone elses house? where you have even less control?
As I said, this is just my opinion, and good luck whatever you do.
Andy