Otherwise, we have always thought our son was normal. He plays sports willingly and has friends. I just don't know what to do with him.
Was the retention explained to your son in a positive way? Children are very perceptive, and even if our words say that we feel good about something they may realize that deep down we may have some negative feelings. Your son may believe that you think there is something wrong with him being in first grade again.
Try talking with him to reassure him that another year in first grade will help him to be a leader in the class and to succeed in some of the things that were hard for him last year.
Talk with the teacher. Together you may be able to make school more interesting by letting him be a leader in activities. He can earn stickers or check marks for participating with the class and you can follow up at home by bringing him to the park or letting him have a friend over to play for good days.
Ask the school counselor to help your son feel positive about school and his retention. Often just a little extra attention from adults can make a tremendous difference in a child's attitude.