We tell our son that "In this culture, that's a word that hurts people's feelings".
We define via culture, because he has some friends from different parts of Africa... in fact, my sister can't come over during any of these playdates... because she is "Reuben-esque", aka drop-dead runway model GORGEOUS in THEIR culture; and it causes friction, a LOT of friction, between the other parents.
We teach our son to watch for the clues that tell when a person is sad/happy/embarrassed/proud ... because following the wrong cultural cues is really really hurtful. (Our line about "rudeness" is that it's a way to verbally hit/kick a person... so if you wouldn't walk up and slap someone in the face, you are NOT rude to them).
Ditto TALK to your son's dad. Many men are often clueless, and still others are just out and out cruel on purpose. Either way, this needs to be dealt with.
(From someone who gained 160 lbs, and has lost most of it, that "gulp-gulp-dieing-inside feeling isn't one ANYONE should feel, especially as caused by the most loved).