The school system offers services for exceptional children and your nephew may qualify for some of them. The staff at your nephew's school or in the exceptional children's office at the school system's central office can inform you about what services are available. Your nephew may qualify for services for children with behavior problems or for children who are "other health impaired" due to attention deficit/hyperactivity disorder.
Many communities also offer programs through private schools for children with behavioral problems. Your nephew's pediatrician or the school staff can help you know where he may fit in.