Check me on this but I beleive in his book "War and Love" he said he was promoted to Captain while he was in captivity.
The book is an outstanding read from beginning to end. It is surprising how much information was passed to him in the letters from his wife and how much he passed along in the letters he send home from prison.
I may have to give it a read. Thanks for the tip. I am not sure when he was promoted, but at the time of his capture, he was the highest ranking officer in the POW camp.