Where was the dog before he had to be inside? If he was always inside the whole time is this the first time it started bleeding? Dogs will not drink enough while it is cold and they lose more hydration with their breath.The dryness affects the capillaries in the tail tip and promotes chapping, making it prone to busting and bleeding. Try boiling down a broiler chicken and saving the broth. Give some broth in the feed each day to encourage fluid intake, if he doesn't want to drink water. Try to Super Glue it. It can help the tail tip, but as long as it is chapped and they have something to wag it against, it will bust.