Thank you for the comments. I can't tell you how nice it is to get such thorough responses.
This week has only gotten worse. I increased Sam's melatonin to 4mg every night and I added magnesium. He still will not go to sleep on his own but he falls asleep quickly. Then he wakes multiple times during the night in a panic that I am not there. With the melatonin increase, he falls asleep again more quickly (before he often would NOT fall back asleep or take hours to do so) but he wakes up again an hour or so later looking for me again. Now, I suppose I could just sleep eith him but I am not willing for a number reasons. With the way Sam arranges himself on his bed, the only way I can lie down is to curl up at the foot of the bed. That is not a great way to rest night after night.
My next step if to try time release melatonin. I also contacted the behaviorist at Sam's school to see if she has any suggestions. If neither of those steps work, I may have to find a way to safely secure Sam in his room and let him cry. I know he CAN sleep on his own and I suspect this is a behavioral phase. It sounds mean but cutting him off is generally the best strategy with him - he doesn't "get" gradual change. I don't know what we will do with Maya those nights but I would rather have her sleep ruined for a few nights than mine ruined for months or years. If we still have no luck I will consider sleep meds. Cross your fingers for us.