makes it sound like he's using slurs;

He doesn't ever say slurs with the intention of slurring a group, and we have successfully stopped him from repeating certain bad words by using the strategy you mentioned of giving the historical context and via the normal parental approach of picking our battles and being extremely firm on him not saying slurs after we have told him that a swear word is an attack on a group. Although anyone wanting to slur an entire group seems so silly to him that it's hard for him to emotionally internalize the harm some people feel at hearing certain words.