I would add to your senior feed 2 lbs of alfalfa pellets and 1 cup of either rice bran or milled flax. That is a simple and inexpensive way to put weight on. Once he's up to a good weight, cut out the alfalfa and cut the flax or rice bran down to 1/2 cup, as it's good for skin/coat/hooves and keeping weight on.
If your senior food is grain based, then I agree on switching him. (check the label and look for corn, barely, etc.) If it is grain based, then just replace it with Alfalfa pellets (3 lbs Am and 3 lbs Pm for weight gain), and add SmartVite Senior
vitamins along with the flax or rice bran. Use some water to make it all stick. Once he's up to a good weight, cut the Alfalfa pellets in half and the bran or flax to 1/2 cup.