Once you have considered the possible causes for his behavior, determine a plan to help your stepson get beyond these difficulties. It may be necessary to reduce the amount of work he is being expected to do or to get him extra assistance, such as tutoring, to help him with the work. If the problems seem to be more emotional, you may want to work out a way to spend more time with him or you may want to investigate counseling for your family.
If you decide that a positive behavior plan will help in this situation, work with your stepson's teacher to set that up. Choose one or two behaviors to work on first and ask the teacher to send you a daily note to let you know how he did. A good day can earn an extra bedtime story 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. As your stepson's behavior improves, you can increase the amount of good behavior required to earn a reward. The key to any system like this is to use it consistently over a long period of time.