When you talk with your child about the retention, focus on the positives. Remind him of the things that were difficult this year and how much easier they will be next year. Help him understand that next year he will be the leader of the class and the one who will be able to help the other children.
Talk with the school counselor. She may be able to give your child some individual time or include him in a small group to help retained children feel positive. If you see that your child is having problems next year, ask the counselor to help him with self-esteem issues.