I make a lot of mistakes.And i hope, someone will point me out to them.Uncle would make him the brother of the MC's mother and his Aunt when multiple times they call him Dad, my father, Daddy....
Uncle is not short for grandfather. Maybe use Papa, grandpa, grampa, or even ogre.
Of course it's a (somewhat) free world and you can call him what you like. I just found it jarring they way you talked about completing the game, yet kept calling him Uncle. Plus, for a guy with potato english you seem to be able to write long complaining posts about character development just fine.
And yes, he is Grandpa, my mistake.