My 11-year-old son has trouble sleeping alone, because of what he calls "nightmares". I don't think they are sleeping nightmares, but rather overactive imagination causing frightening thoughts while he's still awake. His solution is to leave his room and sleep in his brother's room, or with us; when he has company in the room, he is able to sleep.
This has been going on for a couple of years now. At first we thought it was something he'd grow out of, but there's no sign that he is doing so. We have tried to see if there's any particular trauma that initiated this, and there doesn't seem to be anything; nor does he show signs of hiding anything like that.
He was having some stress in his classes when the issues started. He is ADD, and once we worked with the school to accommodate this, the stress has improved, so that he has seemed reasonably cheerful about school now, while the sleep issues have if anything worsened. His daily life is reasonably comfortable and stable.
We have tried many approaches: Sitting with him until he goes to sleep (then when he wakes up at 2:00 AM he goes to another room); lights on while he sleeps; etc. During the school year we are fairly solid with his bedtime routine, during the summer we are flexible; neither approach makes any difference.
We have discussed his fears. He is aware that they are not logical, but that doesn't help. I believe that his thoughts have an obsessive quality to them so he can't stop them. During his ADD evaluations, the psychologists noted that he might be at risk of anxiety disorder as well, but that he wasn't in the range where that would be a major concern. He started Vyvanse during the school year; that didn't affect his sleeping in either direction.
He plays video games, and we've tried stopping them for a week or two; no effect. Games and other screen time stops before bedtime, when he's expected to do something quiet (usually reading); that hasn't helped.
He is physically very active; even on days when he does lots of physical exertion he doesn't get particularly exhausted. It may be possible to tire him out, but I haven't seen it happen. Aside from his sleeping he is very healthy and reasonably happy.
We've mentioned this to his pediatrician, who didn't seem particularly concerned. Before we raise it again, are there any suggestions?