You say that a reward chart has not worked, but try again to focus on the positive. The key to any reward system is to use it consistently over a long period of time. Sit down with your stepson when you are all calm and ask him what rewards he's willing to work for. Keep in mind that rewards don't have to cost money: a good day can earn an extra bedtime story, 15 minutes of computer time, or a walk around the block just with you. Two or three good days in a week can earn having a friend over to play on the weekend. You can increase the time required to earn a reward and tackle other behaviors as his conduct improves.
Talk also with the school counselor. He may be able to give your stepson some individual time or include him in a small group on good behavior. The counselor may also be able to observe your stepson in the classroom and help the teacher see some ways to improve the situation there. Changing your stepson's seat or starting a positive reward system for the entire class may make a big difference in his behavior.