Have you talked to your parents about how you feel and what is happening? What is their thoughts on the matter. it might help if they asked your grandpa to respect their home and not kick their dog.
Being old or growing up in another time is not an excuse. I say that in the nicest way.
EDIT: Sorry I didnt read too well. I see you noted hes forgetful, so maybe some dementia....so Im a little more understanding. But I would still talk to my parents and ask them have a talk with him.