You might try going over his diet with a nutritionist. Does he eat a lot of chocolate? Does he drink coca cola? Do you give him sugary deserts? Even many fruits are high sugar content. Chocolate and Coke have caffeine in them, so if he has a large intake by day, that might affect his ability to fall asleep.
Do you keep to a routine?
Is his room to hot or too cold?
Is there too much light?
Did you try a sound machine?