How does he behave at home? Is he inattentive and restless as well? Does he tend to "space" out? The suggested bevaior management from the previous post is wonderful advice. You may want to take him to a specialist to rule out medical problems. You might want to ask about the possibility that your child is ADHD. Be careful about that, though. Some Doctors diagnose children with ADHD unjustly when all they really need is a consistant disciplanary program at home.
If you read the recent postings on this very topic, and the replies to them, and if you adhere religiously to the methods described, you'll see improvement. When you say that rewards, removal of privileges and use of time out 'do not work', it's likely the problem rests with how you are implementing the behavior management system. I'm going to assume, perhaps unfairly but probably not, that you are not implementing the tactics in a manner that is sufficiently systematic and structured. You want to reach a point, which you can do fairly quickly, when your child will know precisely what will occur when he follows the rules and when he breaks the rules. When such certainty is achieved, you'll see results. Now, he may display some neurointegrative problem or emotional disturbance, but even if he does the behavior management piece of the work would essentially be the same. To rule out the presence of any complicating disorder, you can arrange for an evaluation by a mental health professional.