I know during the summer last year my son would always wake up and have a sweat spot where his head was laying. We helped this by putting a fan in his room and doing what you're doing- no layers or blankets.
Other than heat being an issue, I do notice that now if my son wakes up from a bad dream (assumed of course) or from teething pain and is crying- even if only for a minute or two- he'll be very hot and sweaty when I get to him in his room.
I just summed it up to crying= higher blood pressure and hotter body temp, because if I check on him while he's sleeping he's not that hot.
Try sneaking in during the night while he's sound asleep and feel his head. If he's still really super hot and not crying, then I'd call his doc just in case. It could be just a higher temp from molars, or a slight illness.
Good luck! :)