Does he chew on trees or eat tree bark? Many horses that chew and swallow wood and trees are deficient in Calcium (Ca). This is usually the case with horses that are eating a grass hay / grass diet as opposed to horses eating a legume such as Alfalfa, which is much higher in Ca than grasses or grass hay.
Some horses that are very Ca deficient even lick or eat dirt or lick painted surfaces.
There are very good loose minerals available, usually marketed to livestock producers, that supplement Ca. We use a loose mineral that has 4 times as much Ca in it as Phosphorus (P) and also has 2% Magnesium (Mg) in it. It is only 25% salt, 24% Ca, 6% P, 2% Mg and has 150,000 IU of Vitamin A in it. It prevents or cures Rain Rot and other Vitamin A related problems, stops wood and tree chewing and supplements Mg which really helps with nervousness and behavior problems. They eat it free choice. I have had new horses come in and they ate it like grain trying to catch up on a Ca deficiency.
Before I figured out what caused wood eating, rain rot and lice, I used to cut lengths of cottonwood and willow limbs so horses could eat them instead of my fences. They literally attacked the small limbs and had the bark all eaten off in hours and the whole limbs eaten by morning. Then, I found a Moorman Mineral Nutritionist with a PhD in Livestock Nutrition and have been feeding a high Ca mineral and added Vitamin A ever since.