In children the age of your son, the most common cause of trouble sleeping is actually just problems with sleep hygiene. Teenagers, like your son, are typically starting to have more homework and other late night responsibilities. They also may enjoy staying up later at night and they typically are sleeping later in the morning when they are allowed to.
A major problem is the sleep cycle shock that occurs on the weekends when children are allowed to stay up very late and then sleep in very late the following morning. What this does is reprograms their sleep cycle such that they don't feel tired any more until late at night. This can usually be treated by making sure your child gets up at the same time every day of the week, early in the morning, even on weekends, and goes to bed at roughly the same time every night of the week.
It can also be helpful to avoid all caffeine containing beverages, such as coffee, soda, and energy drinks, in the afternoon and evening.
If these strategies do not work, melatonin, which can be found in natural supplements shops, has been used with some success in children this age.
Please talk to your child's pediatrician
for more specific information!