Something to consider would be an easily digestible, high fat feed like Purina or Nutrena Senior. Start them off slowly, with a vet's advice on quantities to ramp them up. You've probably already heard this, but smaller, more frequent meals will help with the weight gain.
Also Blue Seal Hay Stretcher cubes (not crumbles) makes a wonderful weight enhancing lunch. Be sure to soak in water - if they eat fast and the cubes are dry, the cubes can swell in the esophagus and cause choking. We introduced Hay Stretcher to several t-breds at the barn that have problems with weight and after battling for four years, this seemed to do the trick!