You may be surprised to look outside on a rainy day and see your dog staring in the window or hunkered under a tree in the backyard. He has his own dog house, so naturally you assume he'll use it. Sometimes, dogs need a little convincing to settle in.
Check the placement of the house. While you may want to situate the house so you can see the opening from a window in your home, that placement may not be the best for your dog. Consider moving the house closer to your home, to provide a wind and rain break.
Make the house comfortable. Cover the floor with straw, cedar shavings or a dog bed stuffed with cedar. Don't use hay or old blankets, which can provide a comfortable home for pests.
Entice him into the house. Set his food dish in the back of the house, so he has to go all the way in to eat. Stick your head into the house every day to make sure it isn't damp or taking on an odor. You want the house to be a pleasant spot for him to hang out.
Introduce him to the house when the weather is nice. Don't expect him to hunker down in a strange house in the pouring rain. Let him get used to the house on a pleasant day so he will feel comfortable hanging out there when the weather is bad. Many dogs get uncomfortable in bad weather and are not likely to stay in a strange enclosure in pouring rain or during a thunderstorm.