Just a thought... could it be that he dreamed that he was at the grand Grand Canyon (for instance)? When I was little, even older than your son, I'd have very vivid dreams and sometimes have problems distinguishing them from reality. Even as an adult I'll have a very vivid dream and later on not be sure if that was a memory or a dream (granted, it's normally over something trivial now but I could see where a kid could get confused or misrepresent what happened).