He has no legal authority.
Not only does the federal right to try act allow doctors to prescribe medication for off label uses, but a federal court ruled in another case that a governor does not have the legal authority to ban any drug that is legally approved in this country.
"If you will not fight for right when you can easily win without blood shed; if you will not fight when your victory is sure and not too costly; you may come to the moment when you will have to fight with all the odds against you and only a precarious chance of survival. There may even be a worse case. You may have to fight when there is no hope of victory, because it is better to perish than to live as slaves." - Sir Winston Churchill