Cheeky or funny?

I agree with the above - it’s not all right to comment on body parts and that needs explaining and why. With my dgs2 who was more obsessed with this issue, I also corrected the words he used - ‘they’re called breasts’ I said firmly ‘ if you do need to refer to them’. That’s all part of the respect issue

I think Dad needs to have a bit of a talk with this young fellow and explain how to use respectful and appropriate language - not just to his grandma but to everyone.

At 8, he possibly doesn't know that what he said is impolite but that's why he has parents - to teach him.

We do not usually, in polite society, talk about the size of any body part. It would be just as rude if he'd said you had big feet, nose or ears.
But he needs to know now before he gets himself into serious trouble.
Already at my grandson's school (he is 9) girls are reporting boys (and vice versa) for sexually inappropriate speech and harassment.

He's 8 now, and needs to begin to learn appropriate comments or he'll get into trouble! Your son 'told him off mildly' I'd say that's probably the best way to handle it, depending quite on what was said.
I think in some ways, it's best coming from you (not criticism, always difficult in the moment) as it gives a stronger message. In similar situations I say brightly 'I know, but it's not polite to say so' or 'people /women don't like it if you comment' (actual size really not important!)
You an laugh, and get the message across.

Dgs once asked why I had big boobs I told him it was so I could give the best squashy hugs ?

Kids don't need a licence to speak....they come out with the sweetest of things, sometimes embarrassing , sometimes cheeky , but always lovable. That's the innocence of childhood. And that's why I love holding a conversation with them.?