Talk with the teacher to find out what he or she has noticed. Ask the teacher to help you focus on the positive so that you can help with your son's motivation. The teacher can send you a daily note to let you know how his day was and you can follow up at home. A smiley face for the day can earn extra computer time or an extra bedtime story, and three smiley faces in a week can earn having a friend over to play on the weekend.
You may want also to rule out a physical problem. Sometimes a sudden change in a child can be related to nutrition or an illness, and you may want to talk with your pediatrician to make sure this could not be part of the problem.
Talk with the school counselor. He or she may be able to give your son some individual time or include him in a small group. The counselor may also be able to observe in the classroom to give you an objective opinion on what may be happening with your son.