My DS is 4yo. I just started to teach him what is healthy(to eat), and what is not. He never been in a fast food restaurants before, but saw his cousins eating it a lot. I explained to him that's this is unhealthy food, and we don't eat it. He was fine with it.
Today I didn't have time to cook anything at home, we were outside the whole day, so I decided to buy him Burger King that once. We went to BK and there was this really fat cashier, she asked what we wanted to eat. My DS looked at her and said:"Mommy that lady is really fat, is it because she eat a lot burgers?".I didn't know what to do or say. I was so Embarrassed. I run out of BK and started to explain to him that its not nice to call people fat. He replied:"But Mommy she WAS fat.What do we call fat people? I didn't really knew what to say.
Teach him it is not ok to comment on how anyone looks in front of them. My ds did this too in a grocery store when he was 4 and I was so embarassed but the lady whom he was referring to was way more embarassed.