Talk with your son's teacher about working out a system for your own follow-up at home. Ask the teacher to send you a daily note (it could be as simple as a smiley face or a check mark) to let you know how his day went. A good day with a set amount of work completed can earn an extra bedtime story or a walk around the block just with you. Three good days in a week can earn having a friend over to play on the weekend. The key to any plan like this is consistency over a long period of time; rewards must be given every time your son earns them, and this is not a system to try for just a week and give up.
Also, talk also with the school counselor. She may be able to give your son some individual time and talk with him about why he wants to go back to kindergarten. The counselor may also be able to observe in the classroom and give you an objective opinion of what is going on with your son.