this is more of a tactic to handle your neighbor than the pup itself. Though there are no one particular way to get this problem fixed, I am sure, you can try these things out. Probably when they come out for walk, you can casually walk out and start petting him and initiate a casual conversation where you can talk about the right ways of keeping a dane puppy. Also you can refer to some experts who can offer free advice. Its only by educating the Pet owner, this could be done. Your neighbors dog is not the only case, there are several good dogs which are not being maintained and cared the way they ought to be.. The only effective solution that can be nearest to success is creating and spreading awareness about handling them. Good luck to you..