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.)
