As so many have stated, this is your son's fault. He should take responsibility. And as his mother, it is your responsibility to see that he does. If you fix this for him, you're just teaching him that nothing is his fault.

I have a 14-yr old son & I monitor his computer usage & his cell phone. I pay for everything in the house so there is NO expectation of privacy. And I'm very open with him about that. I don't sneak behind his back - I hold out my hand for his cell phone to see who he's calling & texting. I randomly check the computer to see the sites he's visiting. And after a particularly offensive site was found on his computer, I moved the computer to the living room so I could see him when he was on the computer. The computer stayed in the living room for more than six months & he's very careful now not to abuse the privilege of not having me look over his shoulder all the time.