It's not the PC's job to deal with another disruptive PC. That's a job squarely for the GM.
This doesn't need to be antagonistic or dramatic. It just needs to be the original OP sitting the problem child down and laying it out with the fact the details of the campaign were laid out, if that doesn't suit the PC that's fine he doesn't have to play if he's bored. The OP I'm sure doesn't want him to be bored, but at the end of the day the GM is allowed to run the campaign they want to run, it might not suit everyone's tastes, no hard feelings. The OP is trying to avoid the conversation and decision I'm sure is already painfully obvious to them, which is why they likely posted here.
I think it's important to stop this now and not let it continue. It's wasted GM time not focused on the game, there are apparently other PCs content with the direction of the game, and letting conflict or contrasting views simmer and persist typically only ends badly, which could spoil it for the other PCs happy with the game itself. Best to just nip it in the bud, put that foot down, and end it.