J.S.
My ADHD daughter has always had issues settling down for sleep. Always. ALWAYS. She's been on Concerta for several months now, the extended release tabs, with excellent results. If she takes it at 10:30 AM or later she'll need 3 mg of Melatonin to settle for bed but if she takes it between 7:00 and 8:00 AM (getting together time and leaving time for school) it's perfect and 99% of the time she doesn't need Melatonin. Prior to the Concerta she would frequently ask for half a tablet of Melatonin (1.5 mg).
Melatonin comes in 1 mg, 3 mg, 5 mg, and 10 mg tablets. You can also get it in liquid form. Children shouldn't take more than 6 mg and should always start with the smallest possible dose and build up only to the smallest dose that works for them. It also should only be used under the care of a doctor, especially when in tandem with other medications even though it's a natural supplement. I'd rather not give it to my children at all, and luckily we've virtually eliminated it for my ADHD kiddo.
Have you had your son evaluated by a pediatric neurologist that specializes in ADHD and ADD? They're often much more knowledgeable about the ADHD medications than psychiatrists are because they're so much more, well, specialized in the field. They also will insist on frequent and regular check-ups aka mini-physical exams every 1-to-3 months to monitor the medication effectiveness as compared with the child's sleep patterns, diet, height, and weight as well as physical and mental wellness and activity.