Extremely rare case had to read this one on the internet myself.
Well if it was 1hr. since he took the tablet to washing his stomach then it removes only 50% and if 2hrs. it only removes 15% of tablets so this is pretty useless but still common in ER's in pakistan. As far as the management is concerned IV drips and everything even if this was my case I will even not know what to do as this is so rare. The thing to do so far as I have read is do NOTHING.
Not to worry. Sleeping is only prevented in patients with head injuries and those with benzodiazepines or barbiturate overdose as it can cause respiratory depression. But as melatonin cannot cause respiratory depression so it is not that dangerous. Obviously he will be drowsy for some days, other than that nothing to fear. The only problem to look is seizures(fits) if he is having one take to emergency asap. Some literature to read and to relax you:
Can You Overdose On Melatonin? A Look At The Side Effects And Dosage
Should You Worry About a Melatonin Overdose? How Safe is Melatonin Really?
Melatonin
Problems in assessment of acute melatonin overdose. - PubMed - NCBI
Brother please do not mind, a sincere advice, try to keep medications out of his reach a very high place or lock the cabinets properly. Your child is hyperactive than usual and so you have to be extra vigilant for his safety.