I'd follow up with a simple "are you going to do X or not?", ignore the 'whatever' and move on to getting the task done.
Kids tend to want to try out the stuff we get upset about. This is going to happen with bad words, irritating habits, etc. If it were me, I'd focus on the most important thing in that moment-- what needs being done. Don't get stuck on one word, instead, address the actions of doing or not doing. Getting caught up in words sometimes gives children a lot of power when they press our buttons.
Also remember that, at school, he is having to behave and be respectful to the teachers. If he's only misbehaving and throwing an attitude at home, I wouldn't be overly worried. If kids are doing what's asked at school all day, they are likely tired and grumpy and not wanting much direction at home. Esp. in Kindergarten and the younger grades. Not making an excuse, just putting it in some perspective. If he starts acting out and being rude at school, of course, then yes, more attention to the situation is needed.
And I'm not saying 'let your child speak to you in a bratty way', what I am saying is choose your hill to die on carefully. If he doesn't feel he has room to express negative emotions at school right now, then expect that some of it is going to come out at home. (I see this with my child who is eight-- beginning of school year is still an adjustment. They want to please the teacher and do really well at school, so tired, cranky, uncooperative kid at home. It happens.) If you don't overreact, eventually he'll find something else which he thinks is novel and you think is annoying!
( I would also add, yes, addressing it directly works for some kids. It doesn't work for mine. Not because we don't follow through on discipline, but because he LOVES a reaction and for him, working through/past an annoyance sort of bursts the 'oh, let's push their buttons' desire for distraction my kid has and keeps us on the main point.)