I have an 8 year old granddaughter. She is flexing her independent muscles, as they do at that age, and will sometimes be rudely impatient/smugly rude/shrugging her shoulders dismissively etc. . I tend to approach it as she is perfectly old enough to understand what is rude and what is not. Depending on scenario I might look at her and say "You know, when you don't understand/are being slow..etc etc ...I always wait/listen/explain/ etc etc. That is the right thing to do isn't it!" No further comment. It works!!
I might also say, depending on scenario, something like "Did you mean to be rude then?. I hope not because that would be a pity!". I then quietly get on with something, and before long over she comes with a question or to help me or whatever and more often that not she will "Sorry I was rude Nanny" but if she doesn't I just leave it anyway because the lesson has been quietly learned. There was a spate of these incidents for a while when she turned 8 years, now very rare.
Maybe grandad could also utilise similar tactics. Not a lot of fuss, just gentle reprimand that hits home! And if you do the same without emotion or a big deal, t might just work