K.U.
I think the less reactive you are about it the better, for the most part - if he knows saying it will get a big reaction from you, he is probably more likely to keep doing it. But I would not totally ignore it either. My daughter has the type of personality that she may be strong-willed and stubborn at times, but she also wants to please and do the "right thing" - sometimes she has said some things that I don't want her repeating and I let her know very firmly that that was not allowed, and made it very clear that I did not want to hear her say it again because it is not nice. She seemed to realize that she had crossed a line and knowing where that line is, she has not tried to cross it again.