Beaver.... Or Boy??

Went to strip Dominic's bed today to wash the sheets and found chew marks up and down the inside of his bed! We just moved here in January and it was fine and since it is the top bunk you don't see it when you walk in. He has completely ruined the the bed rail. I asked him why he did it and he just says, "Cause I did." What do I do with this kid!