You can probably teach him a BE QUIET command, but you are going to have to be really persistent and consistent about it. That is where most dog training breaks down, because some people let their dogs get away with undesirable behaviors sometimes so the dog keeps trying because it never knows when you might give in and therefore the behavior is never extinguished.
It almost sounds like he is simply announcing his presence to everybody so all the predators know he is there and will stear clear."I'm here, my territory, trespass at your own risk!" Kind of like a rooster. Its really what you got him for, but I can see how it might be really irritating at that time of night.
Are you sure nothing is about that he needs to bark at? Maybe he can hear something you can't see? Like coons in the woods or something?
Does he bark at other times during the day when you KNOW there is nothing to be guarding against?
I'm asking this because I want to be fair to the dog. If something MIGHT be there you shouldn't extinguish the behavior because you want him to guard and it is his instinct to do so. If you know for a FACT that there is no reason to bark, then you can try to extinguish the behavior.