How old is the dog and has a vet ever looked at its jaw?
In Boxers, jaw set and amount of underbite is directly related to the fact that humans have selectively bred for a shorter, upturned muzzle. Breed standards do set certain proportions for the head and muzzle that breeders try and shoot for. Depending on how well a breeder has selected for head/muzzle shape and proportion you might end up seeing different or more exaggerated underbite and so that might be one reason the lower teeth are showing when the dog's mouth is closed.
But another thing that could cause a jaw set that results in the teeth showing or sticking out when the dog closes its mouth is a condition called "wry mouth". It is a condition where one side of the lower jaw grows at a different rate than the other, causing the jaw to twist or grow awry. Wry mouth is hereditary and dogs with wry mouth have a greater chance to have pups with wry mouth and therefore shouldn't be bred. Wry mouth usually shows itself by 1 year--when it is noticed depends on how severe a case the dog has. The more severe the twist, the more obvious it is and that can mean it is diagnosed early.
Wry mouth can range from mild cases where the dog just ends up going through life with a crooked, toothy grin, to more severe cases where the bite is so far off that the teeth don't line up and the dog has trouble chewing and eating. The teeth end up wearing unevenly and because the dog doesn't chew correctly it can be more prone to dental disease. Sometimes wry mouth can be so severe that the dog's tongue ends up hanging out of its mouth all the time and cause the mouth to become dried out and the dog might need its tongue and gums spritzed with artificial saliva a couple times a day.
He's got a cat-like ability to turn his head around 180 degrees. YIKES!
