You mean the shorter man in civilian clothes, who is not saluting? If so, probably not Robert McNamara who at the time was a serving officer and would have been in uniform. And, whose job at the time had nothing to do with Los Alamos, and at the time was not famous or important enough to be at such an occasion.
It's just a resemblance.
This is a photo of the Army-Navy "E" award ceremony at
Los Alamos on 16 October 1945, the last day of Robert Oppenheimer's tenure as director. The event was filmed; see also. The tall civilian to the left of the flag pole is
Robert Sproul (the then president of U.C. Berkeley), the tall man at the extreme left is Robert Oppenheimer. It is clear from the film that there are two rows of seats on the podium, and that the subject of this question is seated at the last chair of the second row of seats, at a moment when Oppenheimer has removed his hat during a salute, when everyone is standing.
I had thought that it might be Harley Wilhelm, whose parted hair looks a bit like your subject's, and whose age seems closer to the mark, but his award was made on 12 October in Iowa. Maybe its Norris Bradbury, who became the second director the following day. Norris had left active duty in the Navy to be able to take this post. In the film one can see the subject receiving a handshake from William Parsons, the naval officer just to the right of the flag pole just minutes before Oppenheimer received a diploma from Groves.