the thing is you can just SEE what's going to happen a few months down the line - your DS will encounter another (probably smaller) baby, this baby will do something he doesn't like (pinch, bite, grab a toy) and your DS will retaliate by doing the same thing back but harder.
And no amount of telling him it's not ok to give tit for tat when the child is smaller than him, will counteract the example of his granny pinching HIM back, even though he was smaller than her.
It's storing up trouble on so many levels - but if your mum can't see that I don't know where you go from there.