Your story is so similar to mine, though mine is two and has not changed. Totally destructive. Even though he has had lots of training, gets mental stimulation, and goes to the dog park every day, he is no longer fun to be around. My brand new wood floors and furniture are ruined, my car is scratched up and dirty and he refuses to come to me. It is cold here, too, and when I took him to the park yesterday (all ice, snow, mud and cold water) he would not leave…even after all the other dogs left. The only way I caught him was that his paws were beginning to freeze and he could only keep three on the ground at a time. It was dark by then, I was nearly frostbit myself.
I have had several GOOD dogs in the past. This one has been to the "best" trainers in town, has a great vet and a nice home.
I don't think it would be ethical to adopt this monster out, so I am considering putting him down myself and cut my losses, both emotional and financial.
I just wrote a note to the breeder to see if they have similar problems with the dogs they sell.
I had such high hopes.