Ask him to give you (the HR/Talent pro) veto power on any hire you think isn't a cultural fit.
That's veto power even if the business leader wants to hire the person. It used to be called the "nuclear option" during the cold war. Of course, you'd exercise that power with care. You'd push and prod just like Reagan did back in the day, but you'd stop short of using it - most of the time.
It's called leverage. You. With veto power over anyone who would be a net negative to the culture.
That's the test to see if your CEO is serious, or if he just read that culture is important in last month's issue of the Harvard Business Review. Not that there's anything wrong with that...