Have her make sure that she's feeding enough of the senior feed. Check the bag's recommendations and weight out what she's feeding him. She may be shorting him!
If he's getting enough, then I would add two things to his diet, flax meal and a digestive aid. Flax is great for weight gain and it puts a nice shine on their coat. Horsetech.com has a good product called NutraFlax and they can add in pro/prebiotics and yeast to aid in digestion. You want a 4 oz serving of flax for weight gain.
Many feed stores and tractor/farm supply stores carry Omega Horseshine. It's a fortified flax meal that's good. Feed 1-2 cups daily for weight gain.
For digestion, check out Source Focus SR
, SmartPak's SmartGain 4
, or SmartDigest
. All three of these are good choices for helping weight gain and digestion.