If they're actually attempting hibernation, their stomach will feel cold to the touch. Sometimes if they're heading in that direction, but not quite attempting hibernation yet, it will be like what you describe - not exactly cold, but not toasty warm either. I've read a couple people saying that some hedgehogs just seem to generally feel cooler to the touch than other hedgehogs, even when they're perfectly fine, so it's kind of an individual basis thing. If your hedgie feels cooler than they normally do, especially if you know the temperature of their cage is lower than normal, then respond as you would with a hibernation attempt (get cage temp up, warm hedgie up, make sure light schedule is correct, etc.).
I would also recommend getting a CHE set up to make sure his cage stays in that 78-82* range that he's used to, to avoid problems in the future.