I heard of a dog in our local dog park who ran into a stone bench so hard he broke his skull and died, so please try to teach your dog not to do that. If you reward his head hitting behavior by taking him for a walk he will keep doing it. Try to teach him something else instead, easiest is that he has to stop hitting his head, give you a paw and only then do you take the leash. Eventually (hopefully) he will paw at you instead if running into stuff when he wants to go out.