I agree with most of the above suggestions.
Senior feed that is complete is one thing you can do & check the horses teeth.
I would try a different kind of hay, perhaps second cut. Second cut is richer and easier to chew however, I wouldn’t suggest changing everything at once. You don’t want to create more problems. Changes should be gradual.
Too much grain isn’t good so a senior feed with a little beet pulp and soaked hay cubes is a good way to go. I also feed my horses probios every day…it helps with digestion and the food is absorbed better. I had a 34 yr. old pony that loved wheatgrass! It was the only thing that got her to eat anything again after she stopped eating. Good luck!