If it is cribbing, which is wind sucking, then the weaver miracle collar works wonders!
If it is wood chewing, then quitt helps to slow it down. When I used quitt it helped them to stop chewing in the stalls, but if I tied them somewhere they were right back at chewing at the post they were tied to. If it is just pure bordem causing the chewing, I would find him something to do to take up the excess time. Maybe extra hay, a toy, more workouts...etc
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"