Hopefully the chewing thing passes as they age, I know how frustrating it can be as my one dog ate my window frame and part of the wall around it
I kept my dog crated until he no longer chewed anything, it lasted until he was just about a year. He's no mastiff though, he's a boxer x.
I am working in a doggy daycare currently and you would be surprised how many people bring their dogs everyday... I know I was. The one I am at is more of a free 4 all style. It's a couple of size specific indoor/outdoor parks. It allows the dogs to burn off a lot of energy because they can play and run with other dogs. Negative side is that there can be injuries, whether from rough play or a full out fight.
The daycare I am at charges $20 a day, with a discounted monthly rate. But still, expensive. Too much for someone like me. I personally would try everything else before a daycare just because of the cost.