You have a couple of things you can try. Eating tails is not normal. It is a form of 'pica', where strange cravings lead to eating strange things -- like wood, dirt, manes and tails, leather, etc.
You can cut your horse some tree limbs that he will eat like candy. Horses will nearly always leave any other snack to eat freshly cut tree limbs of a soft wood tree. Before I figured out how to feed a good loose mineral, I had a lot of trouble with horses eating tree bark and corrals and barns and had several that would eat tails. . I noticed that they preferred Willows, Cottonwood, Poplar and other soft-wood trees. I could throw out a limb and all of the horses just ate them like candy.
I finally started feeding a good loose mineral that is particularly high in Calcium, Magnesium, and Vitamin A.
I have not had a horse eat trees, barns or tails - anything like that since I started feeding the mineral -- free choice.
It would sure be worth a try.