What exactly is he doing?
And, while we are at it... What is his diet? How old? Gelding, mare, stallion? Does he have company? All these things can play a role, so please answer all the questions with as much detail as you have.
First youtube videos of cribbing and see if that is what the horse is doing. Big difference between chewing wood and cribbing. If it's cribbing you can try a cribbing collar, needs to be very tight and stay on all the time. And you may want to consider keeping him or her out of sight of any younger horses, they can pick it up from watching. I was very upset when my boarding stable moved my 3yo next to a cribber and he ended up cribbing too within a couple months.