What a cutie. I would, if all else fails, call the local sheriff's dept and ask them for contact information for the local game warden/conservation dept. emergency number. They have to have something on record to handle calls like yours. In the meantime, some game bird grower crumbles and water should tide the little one over till you can figure out what to do with him.
If you find it is illegal to keep him, you might be able to find a wildlife rehab. person that can take over for you.
No sign of the parents? Canada Geese have been known to nest a distance away from water and walk their new babies to the nearest lake and or pond. There is usually water nearby when they nest so you might want to check if you have a pond or lake nearby to see if there is a family there with chicks the same size as the one you found.
I think as far as your grand baby is concerned, you might find yourself looking for a couple of new family members in the shape and form of a pair of Pekin Ducks to replace the little gosling if you have to give Lemonade up.