Just smack him hard and say "No! Shut up!!"

I'd go even further with the empathy discussion, but I'd do it in a way that won't make him feel defensive, i.e. not "how would you like it..." and not even "imagine you are that person..." but something like this:

Imagine you are playing on the ___ and you fell and broke your leg....(story of cast etc) and you couldn't run around....and you found yourself gaining weight, and then no matter what you did to try and stop it, you were fat. Then imagine you hobbled into (favorite) restaurant, and someone looked at you and laughed and said, "oh, wow, look at the fattie kid! ha hahha!".....

PP here... I'd also tell him what word is a kind way to describe ppl who are overweight. I think we unintentionally do our kids a disservice by not talking about differences in body size. In our house, we use slim, medium and plump. They know not to "call" anyone by those names, but if they need to describe something, they have words to use. I've noticed my son avoids calling anything FAT now, i.e. I once said "look at those big, fat, juicy peaches!" And he said, "don't you mean plump?!"

Get some books about how everyone is different...

Not only would I ask him, "how do you think it makes that person feel?", but I would make it personal, "some of our favorite people are plump, Mrs. So and So, Grandpa, even me. We don't want to make anyone feel bad about how they look. It's what's on the inside that counts"

Did you try to find out where it might be coming from, specifically?

I don't know... Fat phobia is pretty endemic. I remember by 1st grade definitely feeling like everyone thought "fat was bad," but my parents never commented on peoples' bodies (and still don't) and I don't remember any specific friend or adult saying anything. In other words, I don't think you will necessarily be able to pinpoint and remove a single bad "source" of this message, but instead will have to more broadly teach your kid to be kind (or at least polite).

This is not uncommon for a kid that age. I would just talk about how the other person would feel in that situation and how we try to be respectful of people's feelings and their differences.

Anonymous wrote:
I would bring up the subject again, unprompted, because it bears repeating that some things are not said in public, whatever your child might privately think.

This is a good idea. I should bring it up out of the moment.

This happened to me at one of the area country clubs when I was pregnant excerpt the kid was more like 7 or 8. He was by himself at least when the taunting started so wasn't trying to impress anyone.

Did you try to find out where it might be coming from, specifically?

You did the right thing. Just keep emphasizing that differences are ok. Point out his own if he has one so he can relate (i.e. we don't make fun of people with freckles, or short people, or red heads, or overweight people). It's unkind. People come in all shapes and sizes and they all have feelings.

Did you punish him?

I would bring up the subject again, unprompted, because it bears repeating that some things are not said in public, whatever your child might privately think.

He knows how to get your attention, doesn't he?

My 5 year old has twice in the past month basically pointed and laughed at an obese person. IDK where it's coming from. We don't make fun of people or talk about body size. I'm overweight myself.

Both times I took the kid out of the room and talked about how you don't ever say ew or make fun of someone for how they look. How would you feel if someone made fun of you because you have brown hair? I'm very disappointed. Etc.

Ugh. Other ideas?