The R slur from a person with autism’s sister

I didn’t know where to write this but my brother has high functioning autism and he on the daily calls me (I’m not autistic) the r slur, I take some offence to this not because it means stupid and I don’t want to look stupid but it has such an awful background that I want everyone to move on from and just be respectful to The disabled. He uses the fact he has autism as an excuse I’m not sure how I feel about it and I don’t know if I should say hey maybe use a different word or let him say it willingly. :) asking for a friend