should I stop my 8 yo ds to say "on man", "what's up?"

Thank you all for your precious insights! They all are very helpful to me, at least I know how should I respond to my ds when I hear him say ” Oh man”. I honestly sometimes do overreact to what he said, I need to have more calm voice instead! Thanks Tristan, I should be more careful on picking something that involves moral issue than just my personal preference.