Well, if you are sure of the time log, then I hate to say it, but have you broken the skateboard yet? Don't make empty threats, it only teaches him to not take you seriously.

And in the future, try not to make threats that damage property. ;D Make the threat match the violation -- he looks at sites you don't agree with, he loses his computer priveleges or has to be supervised (and at a time that is convenient for YOU. No guilting about "but I have to get ____ done", HE broke the agreement, right?)

At eleven, the most important message you can send to him is that you want to trust him to be responsible for himself. It's important for you to understand that in the teenage years, you just need to hold tight for the ride and be there for guidance. You can never have enough "control" over a teenager to keep them safe, you must teach them how to keep THEMSELVES safe. That won't happen in a lying/untrusting relationship. He needs to feel that he can come to you for advice and be awarded some respect for having the guts to do that.

For the porn situation specifically, tell your son (CALMLY) that you know he was less than honest with you about his computer ventures, and that you would like to be treated more respectfully and told the truth next time. Don't respond to any objections at this point, wait until he is quiet if necessary and then continue. You understand that he is trying to learn what it is to be a man, but while developing sexual feelings you feel he is looking in the wrong places. Explain specifically what makes you uncomfortable with porn -- that real women will require his respect, will not have balloons on their chests, will not play stupid for him, submit themselves to uncomfortable acts, rip all the hair off their nether-regions etc. etc. Ask him plainly if there is a respectable male figure (dad, maybe) that he would feel comfortable having a chat with about male sexuality and relating to women. Then follow through by arranging it.