FYI on U.S. Political system
As chief executive, the President of the U.S. does have the statutory authority to order the FAA to ground any plane he wishes. Without good cause, it could be viewed as an abuse of his power and overruled by courts but that would take time. However, in this case, he could probably get away with it in the 'public interest' no matter what the 'experts' at the FAA (Boeing) think.