My dog Xanadu chews on absolutely everything from shoes, to DVD cases, to the walls and I don't know how to stop it. He's about a year old (we think) and we think he's an American Staffordshire Terrier mix. I've tried puppy proofing everything but he still manages to find things to chew on like the moulding at the base of our walls. The problem is he doesn't do it when he knows I can see him. How can I stop this ?