Dog won't sleep in the dog house

Zeus is a 100 pounder, he had been chained to this dog house for years, when i got him i also got his dog house. it's been pretty cold here and he sleeps on a couple of old comforters out on the concrete front porch and not in his house, he doesn't whine or bark, but i worry about him.
I know the dogs can tolerate more temp changes better than we think, but it's gonna be in the mid to high 20's and i want him to be warm and comfortable, Any ideas?