Help.. need advice

So my son is 2tears old going on 3 in October 2021. So I started teaching him to call my mom and dad ( grandma and grandpa) because I explained to them that we should correct him. My mother understood but my father got mad for the simple fact that I told him a third time to start correcting my son to tell him to call him grandpa and he said he will know when he gets older. But the thing is when u tell my son go to grandpa or gives this to grandpa he doesn’t know but when you say go to daddy or give this to daddy he knows cause that’s what he calls my father since he was a baby..

My question is how can I tell my father to correct him without him getting mad again and honestly it’s starting to make me mad cause if I tell my son “ grandpa “ he tells me no and say daddy. Another thing that gets me mad is that my dad would tell my son “ give daddy a hug” and I’ll quickly say grandpa but then u see his mode changes and Honestly I understand his my father and all but im his mother I feel like my dad should understand and be okay with correcting my son and for him to stop using the word daddy when speaking with my son.

Any advice?