I'd try to figure out if he's tired enough to fall asleep at that time. Neither my 6 yr old (ASD) nor my 9 yr. old (typically developing) could fall asleep at 8... it's still light out! other things to consider: is he getting enough exercise to tire him out through out the day? does he take any meds that could be interfering with his sleep? I know that if I give Wyatt his afternoon dose of Adderall too late in the day, he will have a hard time sleeping.

You might try some relaxing music. Also, you may wish to ask what the nightmares are about and see if there's some way to deal with them. For example, my neurotypical daughter was having nightmares because of the train that comes through our town at 2 am. Once she told me what was scaring her, I told her that the train uses its whistle because it's a friendly train and doesn't want anyone to get hurt on the tracks. Somehow that took the scare out of the train and ended the nightmares.

It may sound counterintuitive but you may try some caffeine. Sometimes that settles down a child on the spectrum.

The psychiatrist said that for kids, u have to give a higher dose of Melatonin, so I'd check with a psychiatrist about the dose and also the psych says to give it 3 hrs before bedtime. Are u giving a bath before bed? That always helps, no matter what. Just make a pic schedule with reading, then lay down/sleep time. I tell my kids we'll read only so many books then lights out.

I have had this with my son (6) since he was a toddler....we tried and tried the melatonin and it worked at first, but then he just needed something stronger. So we talked with out child psychatrist and he gave him Risperadone .25mg(for about 6 weeks now) at night and it has done AMAZING things for his sleep. He sleeps very sound now, does't get up and get in our bed, doesn't get up and go downstairs and play games or rummage around the house or squirt toothpaste all of the bathroom,he seems to have a better day the next day, and not nearly as many bed wetting accidents as before. He also was seen by a sleep specialist last week (for a possible sleep study) and he said to keep giving him the melatonin but to give it to him at 6 pm and then give him the risperadone a little later. So we have been doing that and it has worked like a charm. Also my kids have to sleep with their door open (closet doors open as well) and a light on...so I bought them these little lights and they love them (they are very cheap so you might go through a few of them, but seemed to do the trick). http://www.amazon.com/gp/product/B002GD16Q6/ref...

but definitely talk with your doctor or therapist and see what the next step might be.

The ped put him on Intuniv to help with the sleep and it did in the beginning but not so much now. He's also on Addreall now.
Our rule is they are free to play/read quietly in their rooms till 10 and then lights out. So we aren't forcing him to sleep at 8pm. But when I get up at 3:30am to let the dog out and I hear "hi mom" when I walk by his door I know we have issues.
Activity level doesn't seem to make a difference. We logged 14 hours of park time with in 6 days one week and he slept the same as week that we were super lazy.
He is sleeping about 4-6 hours a night I think.