Melatonin medication in children, how long does it take to work? ?

My 4 year old has been on the melatonin liquid form for 3 years, he has sever autism and doesn’t sleep!! It’s since been changed to tablet form in the hope of keeping him asleep much longer than the liquid form ever would. However he still woke up exactly the same time as usual at 1am after going to bed at 9pm. I’m so so soooooo tired!! Does he maybe just need a few doses in his system? I really forgot to ask his doctor and to ask her now it would take me a week to get an answer from her. So I’m asking on the off chance anyone on here is on melatonin or their child is and knows how quickly they started sleeping through after starting the new medication