Are there certain subjects that interest your son? If he loves dinosaurs or airplanes, an art project or math/science project related to a special interest may motivate him. You can always ask your son what he would like to focus on in his studies.
Talk with the school counselor. She may be able to give your son some individual time. Most first-graders don't have the verbal ability to tell us exactly what is going on with them, but your son may tell the counselor through his play or his art what is bothering him.
The problem could be as simple as your son wanting to be in a regular first- grade class instead of the gifted program. If you find that he wants to be with kids who are not in the gifted class, look for a Cub Scout troop or a sports team for him to join. Make sure that he has opportunities to play with children in the neighborhood as well.