If you think he won’t do it, you don’t know Trump.
He made Ric Grenell the acting Director of National Intelligence. He made Matthew Whittaker the acting Attorney General. He made Ben Carson the Secretary of Housing and Urban Development.
Sure he might have trouble getting RFK Jr confirmed. But he learned to get past all that inconvenient folderol by making his minions and henchmen “acting” officials. He would be happy to let RFK Jr. do whatever he wants. After all, Trump will never have to run for president again. (Either he’ll be term limited, he’ll just refuse to leave or he’ll die in office.) He has nothing to lose.