S.W.
Hi C.! I personally think that this is a learned phase that he will go through since he is so young. Maybe you can lovingly suggest to him, "Please say Yes Grammy or No Grammy when I ask you if you can hear me". You can also say, "You sound just like Grandpa! But I would like for you to tell me, Yes Grammy or No Grammy when I ask you something."
I know grandparents are sometimes not appreciated for making corrections, but I have found that as long as it is made lovingly, the child will be able to learn more manners and social skills than with just a mom nagging day after day.
And I have also learned that even the most annoying habits will eventually fade on their own if you don't make a big fuss about it.
Best Wishes!!