Let it go, but make sure he knows that's an inappropriate way to talk. At his age, he probably considers it hilarious and will start using it. :glare:
With regards to the pop issue, it may help if you suggest that he asks for sprite, 7up, root beer, or some similar caffeine-free pop (that he likes) when offered a soda. If he's out anywhere and ordering off a menu, they'll definitely have one of the above. If it's at his dad's house, they might not, but if he has a liking for it they might start keeping it around.
Or they might not. But it's worth a shot.