Try filled bones from pet shops, these are robust enough and filled with a tasty filling to keep a dog amused for hours, I found these helped so much whenever my hound was left for a few hours, or a good knotted raw hide chew.... get your dog into the habit of receiving a new tasty chew bone each time you leave him alone, he will get the message that when you leave something nice happens too....good luck, he will grow out of it, patience and perseverance though all the way...