My parents adopted a rescue from a kill shelter a year ago. He was a pretty good size dog, but since, we have realized that he was a huge puppy. We believe he is St. Bernard and Boxer. We have tried several methods of training including a shock collar. I know many don't agree with it, but he is starting to get out of control. He lunges at visitors, does not listen, and uses his mouth quite a bit. He is a sweet giant, but the circumstances are getting over the top. Please help!