My Bull mastiff was the same, chewed up the corners of my ourdoor chairs (tub chairs), so I went out and got him his own - he felt safe in the rounded tub chair and it was either boredom or due to teething he continued to chew it. Sometimes doesnt matter what you do, or how many toys they have, they just want to chew. We have about 30 different toys, and every night we change them up, including kong toys and other treat dispensers and he would still chew his chair. the back of it is almost gone and we used to hide it when visitors came because it looked gross! But he is just over 12months now and has finally stopped and is finally sleeping in his kennel. He's not the smartest dog(or its more stubborness then smarts) but as someone else said here, its not just physical stimulation its mental as well. Mine loves chicken and dried kydney treats, but get him outside where there are other people and dogs and he just wants to play. I do all training in the back yard after I take him out for a walk and teach him to be calm. There are a lot of dogs that come to their fences and bark and jump, Until he is calm I let him get closer (I know the dogs so its not a problem, if you dont know them I'd ask the owners).