Personally, I would give something with high calories and digestible energy, so I would go with alfalfa pellets. Start with 2 qts per feeding. Add some water to mush them up (not a lot) and then add your senior feed. To that, add a probiotic like SmartDigest and a good fat source like Flax Meal. Ditch the Cool Calories, it's just a solid form of vegetable oil. Corn or Veggi oil doesn't work well and can be not good for senior horses. I have had better luck using flax. Horsetech.com has a good one called NutraFlax. Their prices include shipping.
Start with 1/2 cup a day of flax meal (2 oz by weight) for 3-4 days, then increase to 1 cup. Stick with that for a couple of weeks, with your 2 qts of alfalaf pellets twice daily. If you don't see much improvement, increase to 1 cup of flax twice daily.
Also make sure he has free choice grass hay in front of him at all times.