i'm a firm believer that 99% of dogs can be rehabilitated. don't take putting him down as the only option until it becomes clear that nothing will work. i'd suggest contacting a behavioral trainer, and see what they say, even though he does listen maybe he just needs a little behavior specific help. does he get enough exercise? maybe he might be getting frustrated / grumpy b/c he has some energy and no way to get it out. have you changed foods or started giving him treats more often? i know when i changed our danes food he had a lot more energy than he did on the other food, maybe something as simple as treats could do this also. maybe you could try some sort of sport, or even just long walks. typically, tired dogs are happy dogs. when he has nipped, was it always centered around his collar? i know we had another person on here with the same problem except it was with the couch and going outside, and it was brought up that grabbing their collar is a very dominant gesture. maybe you could leave a leash on him all the time and grab it instead of his collar and see how he reacts, and if it's good praise him very much. come to think of it his name was bear too. sorry for the length, and welcome to bpo!