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?

Yeah. General rule of thumb is if it's too cold for you it's too cold for them. But my pup will sit outside in the snow and she's a short hair Shephard mix