This website explains the various branchs of the RAF
http://www.rafweb.org/Branches.htm
General Duties was jsut that, general flying activities etc.
What is your airmans name and serial number? You can trace his career as an officer in the London Gazette
http://www.london-gazette.co.uk/search
certain numbers of airmen from a graduating class would be commishioned, it was not based upon duty done before that, some men would be commishioned directly from the rank of Leading aircraftman. I'm no expert now I must stress.
Read around the rafweb.org website it may answer many of your questions.