Find out his interests and look for groups related to them. If he's interested in computers, there may be an interest group in your area or classes on Web design or other computer-related topics. If he's interested in space, there may be an astronomy group or a class at a community college nearby. Your public library staff should be able to help you find these groups.
Talk with your son's teacher. Explain your concern and set up a plan for encouraging him with his work for school. Perhaps the teacher is planning to assign some high-interest projects or could help your son learn more on his own about an area in which he is interested.
You can also control your son's TV and video-playing time. Limit the amount of time each day that he spends on these activities and insist that he do something else when the time is up.