wood chewing is different from cribbing
wood chewing is usually caused by a mineral dificiency - you can put him on a supplement called Quitt (guaranteed to work) or you can paint your fences with used motor oil + cayenne pepper (I think Farnam also makes a product called NO CHEW)
Sorry, kickshaw, I don't agree. Wood chewing is NOT usually
caused by a mineral deficiency. It can be, but it's a habit, like us chewing our nails. Habit, boredom, attention-getter, copy-cat, fear, insecurity...
And DEFINITELY DON'T USE MOTOR OIL on the wood! This is a poison and can cause cancer and who knows what else. A bar of soap, as mentioned, spices, like cayenne mixed with the soap, sunlight dish soap, etc. are all possible things to make the wood less appealing.
I have had three chewers -- I cured all of them, even one that decided to try it after a new horse came in that chewed. I put sunlight dish soap (diluted with water to spread well) with a LOT of cayenne mixed in on every stitch of wood I could find in the barn. I made sure there was good-for-them poplar trees / logs to chew on in the paddock and after they settled in to their new home, they all quit chewing on the barn.