I have no experience with horses than cribs, but one of my grandmother's horses it seems is literally killing himself. I mean when they got him he was sold along with his little brother (2 and 1 year old). By the time they were 3 and 4 little brother was almost a hand taller and filled out beautifully. And the cribber Bart is his barn name, was barely reaching 13.2hh (his parents are 14.2 hh and 15hh, and skinny. I just say him again (5 years old) and I was in shocked there was nothing there, bones every where and he had ate his stall (this are the same people that just made a brand new barn), and than he ate some else stall! He can't be trusted with other horses because he goes into a frenzy with flying hooves and teeth. Not only that but he has become spooky as ever, tense, and doesn't learn well, like it took 4 months to except a saddle and a rider let alone anything else. They give him sweet feed to gain weight, but he making everything worst (the owner is clueless when I comes to horses and weight), he stands there and sweats.
Keep in the mind he cribs none stop and ates wood at all times. Is it possible he could kill himself? Because it sure seems like he is doing a good job of it. Is there is anything that can help him? Nothing that tastes bad works.