He didn't know what had happened, about the planes.... he thought it was just a fire. His "oh god" at the very end wasn't very calm, that's for sure. How could anyone possibly wrap their minds around it?
Nobody expected both towers to collapse. Watching the second one fall on tv was like seeing a horrible movie. It just didn't seem real.
It's going to be a long time until I'll be able to get that last five seconds of audio into the back of my memory. Right now, I keep hearing it over and over.
What you say is most likely correct. He could see the blinding and choking smoke and feel the heat. And no rescuers in sight. He probably knew from the conversation with the operator that it wasn't going to happen. Calm and collected might not have been accurate, but I'll bet that most peoples language would be much stronger in that situation.