Hard for me to say where the ownership is. I don't know anything (outside of this article) about this case.
Was he there on his own dime (or that of an agency outside of the government)?
Are all of his photos property of the government or does he get to retain ownership of those photos he "doesn't share"?
I ask this because work for the government carries no copyright. There are very famous photos by famous photographers that belong to the Library of Congress (some under WPA programs). Same with NASA images.
I don't want to see the photographer done wrong. I just have no understanding on what the release said.
He's not on Uncle Sam's payroll (anymore, seem to recall he was in the Army once). I only know what I read in the article and on Blackfive.