Hmm. The cool belly and sluggish behavior does sound like a hibernation attempt. But it sounds like you did all the right things, i.e. warming him up and monitoring him.
Does your hedgie have a heat source for his cage? Space heater or CHE or something? If not that is most definitely something you will need to get to stop any further attempts at hibernation. If you DO have a heat source, you might want to see if your A/C is too cool in his room or make sure there are no drafts coming into the room from windows, etc.
I'm not sure about the dry nose, but if it becomes runny I would take him to the vet as that can be a sign of an URI.
I'm glad your little guy seems to be back to normal! You did a great job, so congrats! =]