Looks like you just answered your own question... If he knows where the water bowl is, I'm sure he is drinking when your sleeping or not looking. If he's acting normally, I wouldn't worry about it.
I actually was worried about this when I got my first corn and I still rarely see any of mine drink but every once in awhile I do catch them at it and they're all healthy, eating and defecating... So I'm not worried about it.