Try it! Our son has severe ADHD as well and can't calm his body at night to fall asleep. By then the ADHD medication is long out of his system and he is just like your son at night ... he just can't get his body to slow down. We tried it all, too, with darkening the room, putting on soft music, keeping it silent in the house, you name it.
Our psychiatrist also recommended melatonin when our son was three and it's awesome! We started small, cutting the 3 mg. tablet into quarters. It kicks in within an hour and our son falls asleep great! He's seven now and takes 1/2 tablet and is doing great. He falls asleep great and is well rested in the morning without being groggy. We love melatonin!
ETA: Since melatonin isn't regulated, there can be great differences between brands. So, definitely start off very small with the dosage and work your way up, as needed. Some brands are pretty potent in small dosages.