Melatonin has been used in kids, only like Altair said, they should have higher levels than the average normal already. Make sure he's not been exposed to screens too much, especially in the evening and at nights. There are homeopathic remedies for children as well. If you try melatonin, I would start with half a milligram for a few nights and see if that helps reset his inner clock.
Another thing is to get a red bulb (red light spectrum), like the ones that physiotherapists use. If he gets exposed to that light for 10 minutes or so in the morning, he should have healthier levels of melatonin at night. It substitutes the red light spectrum that he should be getting from the sun.