Melatonin

Hi Im very new to this and just after a bit of advice. Our son who is 3 1/2 is due to be officially diagnosed next week, we have already been told it will be high functioning autism but need professionals to meet and agree.

He has been on 3mg of melatonin for about 6 weeks now, after the 1st week he slept a full night for the 1st time ever, and has so since only ever waking for a wee or to be "tucked in tight". We really thought we were sorted, his behaviour improved through the day, his nursery school said he was more compliant and joining in with theirroutines a and he seemed happier. All of a sudden he has started with the same old night time performance, tonight is night 5 of him waking being really distressed. He wakes screaming and lashing about, he will hurt himself and if I try to comfort him he lashes out at me- this happens about 4-5 times a night, lasting about 1/2 hour -2 hours a time. 

I don't really know much about melatonin and don't kwhat if 3mg is much at all?

Has anyone experienced this set back, is it normal? Please someone tell me it doesn't last and he'll sleep again! I feel like I'd just got used to getting somesleeps and it's been snatched away!!

Thanks