I think this would fall under a reasonable request by the airlines to protect life and property. He is free to travel, but the airline is free to deny service. Just like "no shirt, no shoes, no service", it is a public health and safety issue.
I understand his privacy concerns, but he has alternate means available for travel that do not require ID. He should be directing his privacy concerns in more productive areas like Social Security numbers. That is the worst kept secret info in the world.