So, DS doesn't have HUGE problems with sleeping.
Melatonin will help him go to sleep, but it won't "keep" him asleep. I would start low, the lowest dose possible and work up from there. My DD takes 1/3 of 1 mg, and it works great.
A child may HAVE ADHD, but it is not what they ARE. Never tell a child they ARE ADHD.
We give my 11 year old 5mg of Melatonin a night and it helps keep him asleep all night. before that he would get up every couple of hours and couldn't sleep through the night.