I don't think it's that easy. They would make him testify in camera.
It’s easy enough for Mueller. It’s easy enough for the FBI. It’s easy enough for the DoJ. It’s easy enough for the State Dept.
POTUS can say on camera “I can’t answer any questions because ... national security concerns. I can’t tell you what those concerns are because ... national security concerns. That will be all.”
Are you telling me that the POTUS has fewer national security concerns than his subordinates or less authority to invoke them?