P.M.
There is no way to prepare children for all the misunderstanding or outright cruelty they will encounter in their lives. But there are some things that will reduce the potential for emotional injury.
One is to simply acknowledge when something is true. My little grandson tells me I look old, and I simply agree. It doesn't hurt unless I'm in denial about being 62 years old. If he lives long enough, he'll get wrinkles, too.
A child at our church is called brown, black, or chocolate by some little white kids, and she agrees with a smile. It's simply the truth, and it has no power to hurt unless she believes it does. There's no shame in being brown, black or chocolate.
I met a young woman who was born with only one arm, and she simply accepts that people would sometimes be curious, stare, ask weird questions, and make awkward comments. That's just what people do when they encounter something outside their usual range of experience.
I know a man who is flamingly gay. He used to bristle when people called him queer. Now he just agrees, because it's the truth. He's a queer fellow. His life is so much easier, because now people can't offend him, and stop trying to attack him verbally.
A man in a wheelchair notices that people avoid looking at or talking to him. He knows that it's often because they are afraid of saying the wrong thing, so he starts conversations himself. He's got a collection of jokes that fit awkward situations, and you can see that it relaxes people right away.
Really, nobody is completely "normal," and if they insult others, it's often because they secretly feel insecure or unlikable, and hope to deflect people's critical attention onto someone else. Suggest to your son that he simply nod and agree when people make observations about him. What's there to fight about, or even be insulted by, if it's true?
In the case of kids saying "si," maybe I'm a little dense, but I don't understand why that would be a problem.
(Bullying will sometimes become an issue, but that should be handled differently, getting the teacher or principal involved in a solution.)