What helped me was to get rid of all of the sweet feed. I have mine on a mix of 2-3 qts of alfalfa pellets, 2-4 qts of plain whole oats, 1-2 cups of flax meal, and a good vit/min mix, plus free choice hay.
If you're already feeding senior feed, make sure you're feeding it to the bag's recommendations. You may not be feeding him enough. To that I would add 2 cups of flax meal (high in fat), a digestive aid, and either 3 qts of alfalfa pellets or 2 lbs of beet pulp.
If you're already feeding the alfalfa hay, you might just increase that instead of adding anything to the senior. Make sure he's getting 8-10 lbs of alfalfa hay a day.
I have not had good results using corn or veggie oil. I would use the flax instead. If you cannot find affordable flax, then stabilized rice bran works well, or Nutrena's Empower supplement (it's flax and rice bran blended pellets).