My guess is that your son is joined by and influenced by many of his peers in this racist humor. It would not surprise me if he/they made similar derogatory remarks about other racial minorities, homosexuals and women. Such humor is not uncommon among white boys of this age group. The roots of prejudice are fear and ignorance. I wonder if he or you have any regular contact with African-Americans. It's much harder to see the fun in racist humor when you have members of the degraded group as friends or significant acquaintances.
Perhaps you could ask him how he thinks he would feel if he heard others making racist/ethnic/religious derogatory jokes about his own family's background or religious beliefs. How would he respond? Would he be silent? Would he laugh? Get a discussion going that doesn't focus on making him feel like a bad kid for having told you these jokes. Focus on getting him to empathize with the targets of his jokes and putting himself in their shoes. Tell him that you know he is a good, kind kid and that you expect more from him. If he persists in telling these jokes in front of you or other family members, consider whether part of the "pay off" of telling these jokes is his getting you frustrated and angry. You cannot forbid him from telling these jokes outside of your presence but you can engage him in discussions, which may result in his thinking more considerately about the harm they cause.
