As long as your son is healthy, it probably wouldn't hurt him to try an
over-the-counter sedative, but I'd suggest you take him to a proper
work-up with a physician.  Insomnia can be caused by a number of
different things, including apnea, which can be difficult to diagnose on
your own.

Pharmaceuticals might be the last thing you'd want to try for it.  Some
things that helped when my then 12 y.o. had an upside down sleep
schedule were exposure of skin to sunlight during the midday hours while
being physically active.  This helps set the biological clock.
Avoidance of darkened rooms during the day helps, too.  Keeping windows
at least partially uncovered so that dawn and morning light helps keep
the biological clock on track, too.

Melatonin once ready to turn off the lights and fall asleep can be used
to reset the clock, too, short term.

Susan

Here is a link to some non-drug behavioral practices for
overcoming insomnia - these were prepared by one of the top
researchers in sleep, Dr. William Dement.

http://www.stanford.edu/~dement/howto.html

FurPaw