I work at a barn with a lot of senior horses and the best, most cost efficient way of feeding is to just send hay through a small wood chipper. They do really well on this chopped hay, and it give them something to chew on. We also give them Empower and alfalfa pellets.
I feed my thoroughbred gelding a mix of beet pulp and alfalfa mini cubes, soaked in water. Does wonders and he loves it!
You can feef him Timothy or alfalfa pellets soaked in water
I feed hay pellets twice a day along with his senior feed, and some Empower Boost in the winter months.
You can also try hay cubes. Make sure they are extra soaked to the point where they are falling apart and almost in a soup, and that you feed little and often as it will be (partially) replacing hay in their diet.
Do a nice mash of beet pulp , some kind of senior complete feed and rice bran, friend has a senior pony with no teeth thriving on this feed.
I recommend feeding triple crown senior as a complete feed. Occasionally I add a bit of alfalfa pellets, if I need additionally filler. But I have kept many elderly horses fat and happy on it for years without hay and it is truly a lifesaver!!!
When this happened with my Arab (he didn't have any back teeth, ulcer issues, also a picky eater), I did a good thing: soaked-to-mush hay cubes & rice bran, plus his needed supplements and fed him 3-4 times daily :)