I posted a similar problem with my daughter. It's really frustating because you can't do anything when you're not there. My daughter has a sticker chart at school, when she finishes her tasks on time she gets a sticker and after filling the sticker chart she gets a prize. It's helping a little, but she still has problems. Maybe you can start a chart with the teacher. His teacher can send him home with a smiley or a star when he's done well for the day, and nothing when he hasn't, then everytime he accumulates a certain amount of stickers he will win a prize from you. Positvie reinforcement seems to work better for my daughter than dicipline or punishment.