I think I understand what Oh mommy! is saying- that your son needs to know that there really are long-term consequences of not completing work or taking no responsibility at school. I'm sure his teachers have been drilling these ideas into him, but he needs to see that what is being said is more than just a few words- it is very real. Perhaps you could come up with a hypothetical story to tell him alongside Oh mommy's idea of physically showing him what he can lose. I think Oh mommy! has hit the nail spot on- your son needs to know the reality behind his choices.
Good luck!
One method I've used to good effect with children I've worked with is the points system.
Your son will need a small notebook, which he can decorate with a theme of his choice (space, pirates, etc.) You can then reward him with points for good behaviour, and remove points for bad behaviour.
Make sure to specify a treat at a certain number of points earned (100, for example), and stick to it.
While the points system is similar to reward charts, the advantage is that you can carry the notebook everywhere with you, so that he can see points being added or subtracted there and then, as a direct result of his benhaviour - and not hours later back at home, when the situation may have changed.
Plus, allowing him to theme his own book will give him something to work for (e.g. for a pirate theme - have him earn 'gold coins' instead of points; or collect meteors in a space theme.)
