How to keep a dog from chewing the crate door.?

Boagie our cocker spaniel, when we leave our house we lock him up in a crate.we do this because he tears up the house if we don't. He has been really great about the crate until recently. He has been trying to escape and got his head stuck in one of the plastic crates and one of the metal fold up crates. He has been using his teeth to bend the metal bars on the doors. The last two time there has been a lot of blood. If he keeps it up he won't have any teeth. What do we do?