Wet his feed down, no matter what it is...
There are lots of manufacturers out there that are currently making "senior" feeds. I would call your local feed store and ask what they carry. Start with it, for convenience sake, but don't be afraid to switch to a different brand if you are not seeing the results you want. The products are NOT all the same.
Two of my horses are ancient - one is 29 and the other is 37. They also literally have no teeth left. Everything I feed has to be soaked in water until it is soft and squishy, and then they "slurp" it up. They have 24/7 access to good pasture and hay, but digest very, very little of it. Most, if not all, gets "quidded" or rolled into fist-sized balls, in their mouth and then spit out.
Over the years, I have tried many, many different feeds for them. Purina, Dumor, Blue Seal, Atwoods, Nutrena, etc. I am currently feeding them Triple Crown Senior and Triple Crown Timothy Balancer cubes. The horses are doing amazing on the Triple Crown feeds and look better than they have for years. The Timothy Balancer cubes are actually a product of Ontario Dehy, and have additional vitamins/minerals compared to regular cubes. I absolutely LOVE them.
Also, you need to be feeding by WEIGHT and not by VOLUME. The easiest way I have found to weigh feed is by using a hanging fish scale that can be found at most sporting-goods stores. Hang a bucket on it, tare it out, and add a scoop of your feed. You will find that a "scoop" of one type of feed can vary greatly in weight from another brand/type of feed. That is why the only consistent way to feed is by weight. Same goes for hay - not all bales weigh the same, so not all flakes will weigh the same.
There are also weight-building supplements available that you can try, but they should not be used as a substitute for poor feed. They should only be used in small amounts - as in OUNCES - to enhance other feeds. I've used Farnam's Weight Builder in the past, and Nutrena's Empower. Haven't had much need for them with the Triple Crown products, though.