His house may be too "small" for him. Unless raised in and around small spaces, most large breed dogs instinctively don't like being in close quarters. My chow (looked purebred even to the black tongue, but was over 100lbs of pure solid muscle) would not ever even consider going into a small, enclosed space, but he loved being out in the coldest weather (then again, cold in southern Georgia is not like cold in say, North Dakota.) We built a large 3 sided, open front tool shed in the backyard that he would occasionally go under in the rain, if he wasn't ready to come inside just yet.
Not sure how to convince him that that little "box" (no matter how nice it is) is a good place. However, you might see if a larger, more visually open overhang that blocks off the prevailing winds may be more to his liking. I've always spoiled all my critters and have never had an animal that I kept outside (not even our 150lb Newfoundland - who also didn't like 'doghouses') so *my* advice would be to bring him on in! ;)