After reading all of the replies, there is one thing that I have not seen mentioned. Check the thyroid. Bloodwork should always be done when other obvious problems have been eliminated. Checking the teeth, fecal exam for worms, testing for ulcers, a comprehensive vet exam are all part of the puzzle to find the cause. Bloodwork results can explain issues not found otherwise.
When I purchased my 17 year old Arab gelding, he came to me on the thin side. His owner said he had just come back from summer camp and needed more food. When that did not work, the vet came and drew blood. Tested for thyroid as one of the tests run. Sure enough he had a thyroid problem that required Thyrol-L which fortunately was and probably still is quite inexpensive.
This worked almost immediately, then all of a sudden he was losing weight again. A second blood test showed the Thyro-L level was too high so we cut his dose in half. Once we did that his weight leveled out.
As he aged and was around 26 years old it became more difficult for him to eat hay…so…I started adding beet pulp without molasses and alfalfa pellets (started with the cubes but they took too long to soak) to his Purina senior grain. The pellets, senior feed and beet pulp would soak quickly in enough warm water to make a soup. I fed him 3 times per day with this mixture, along with hay which he nibbled on. This seemed to satisfy him and keep his weight where it should be.
I would not make any feed changes without consulting your veterinarian, and when doing so, make slow changes over 7-10 days so as not to disrupt his stomach.