He is developing callouses on his elbows, and it's not dangerous. A dog can get callouses on any point where he puts constant pressure, even a dog that stays in the house all the time. You need to add a little Omega 3 and 6 fatty acids to the dog's diet, like a teaspoon of cod liver oil a day in his food, and put something like Preparation H or its generic form, on the elbows to soften them. Do this daily. Cocoa butter is also good, but more expensive than a generic Preparation H. I know it sounds funny, but it works!