Why is my dog eating sticks, leaves and dirt?

My husband is at the vet with our dog who has steadily been losing weight. They are doing a fecal test for worms, but the vet told him she has found pieces of sticks, leaves and wood in his poop. Our dog is an outside dog. He has eaten over half of his dog house. We can't give him a blanket in the winter cause he will try to eat it too. Oh, and how he has developed pus fill sores on his back.