If this is the only problematic behavior I would not advise use of medication at this point. Rather, try to handle it behaviorally, perhaps by utilizing a straightforward incentive system (the reward would be earned by staying in his bed). Often children with such a worry are not able to articulate any specific reason - they
simplySimply sleep feel frightened. So, it's likely he's not holding anything back. In fact, if we press such kids for specific reasons, they'll often comcoct something in order to satisfy the question. Such fears are often associated with children who display diagnoses along the PDD spectrum, but by no means are such behaviors confined to that population. I'd stick with a behavioral approach and see where that leads you. Let us know.
John