I'm fairly certain that in order to determine the correct shoulder patch, you will need to find out how POW camps in the U.S. were administered. The captain in question was almost certainly a staff officer in the intelligence, or perhaps the operations section (if he was in an intelligence battalion of some sort) of a given staff. Therefore, you would need to find the division level or higher unit to which he or his unit were assigned in order to find this out. If you can find this information, I have a book or two with a great many of the division patches from WWII.
If you are really stuck, try contacting one of the Army's museums, West Point, or the Intelligence Corps.
Good hunting!
P.S. Pangaloss is correct - "corps" insignia is worn on the lapel, and doesn't necessarrilly have anything to do with the operational command to which one is assigned. It is more of a professional discipline / MOS sort of thing. The Marines don't make that distinction (Marines are Marines first and foremost, MOS's are a secondary consideration), so my first hand knowledge of how this works is a little sketchy.