Hitting an animal, any animal, is NEVER a proper training method! It will teach the dog that this is acceptable.
When the puppy jumps up, your Grandpa or you can take the pup and hold or place him tummy up. It is a sign of dominance over them. A firm (not hurting) hold on the snout and a look in the eyes reinforces this. It is not mean, it is what wolves and dogs of all breeds have done to teach what is and what is not acceptable.
I did this with a very snappy hight spirit puppy, a samoyed. My dad laughed saying how will you teach her holding her like a baby? I teach her by being the alpha.