Changing to a pelleted ration and, if necessary, moistening the pellets, will allow your toothless friend to continue to thrive. Continuing to feed hay to a horse with badly eroded molars can result in an impaction and a life-threatening colic situation. If you want to continue to feed such an individual hay, purchase a heavy duty leaf shredder and chop his hay. Feeding grain or pellets moistened with warm water can sometimes induce a picky eater to chow down - especially in the winter. Wheat bran is a popular feed additive that most horses enjoy and can increase the palatability of the ration although, like so many things, it, too, can cause problems if used to excess.
Water is another important factor.
Since the horse's ration contains a great deal of dry matter, if you want him to increase his feed intake you must provide plenty of water. Even a horse with a good appetite will decrease his intake if water access is reduced. Provide an older or convalescing horse with a particularly deep and comfortable bed. Chances are he'll tend to lay down more than a younger or healthy horse. If he can't do so comfortably he'll tend to remain standing more and will be cheated of some rest that may help with his recovery.