Your grandfather's gun is most likely chambered for the old .44 XL Shot(extra long) It was a somewhat popular brass cased round, and definitely much shorter than the .410 ga. shell!
The good news is you can make shot cartridges from .44 magnum cases. They will be short, but that wont hurt the function of the shotshells. Speer makes .44 shot capsules, and you'll need to buy some #7 1/2, #8, or #9 bird shot to fill the capsules, then you can seat them over a light charge of pistol powder such as Unique or Bullseye. Speer lists a load of @6.0 grains of Unique with the shot capsules. This will be under 1,000 fps.
BEFORE! shooting any loads in this Iver Johnson, you should have it inspected by a gunsith, to be certain it is in good shooting condition first!