He will likely have a cabinet position if Trump wins, so not sure what you mean.
Maybe he will and maybe he won't. No doubt Trump is taking advantage of the few votes RFK Jr. may send his way next November. Trump expects loyalty while he is notoriously disloyal. I am not sure RFK Jr. will be rewarded with a cabinet position if Trump wins.
And if he does not get a position there is nothing, he can do about it since it is a federal crime for a candidate to promise someone a political appointment in exchange for an endorsement. If push comes to shove, Trump will deny he promised RFK jr. a position in exchange for his endorsement. I highly doubt Trump would be dumb enough to have committed anything to record. But then, Trump has made many mistakes when it comes to illegal activity and has seemingly gotten away with it since he is not locked up... although he has been convicted thus far of 34 felony counts.
Cabinet turnover is notably high regardless of who is President. However, Trump's senior executive staff turnover exceeded that of many former Presidents at 92% by the end of his term. Trump had four different chiefs of staff, press secretaries and national security advisors over a four-year period. During this time period, six or seven communication directors resigned.