Eric,
The Navy Achievement Medal and the National Defense Medal are both post WWII creations.
Link to NDSM:
http://www.afpc.af.mil/library/facts...et.asp?id=7803
Link to NAM:
https://awards.navy.mil/awards/webap...2?OpenDocument
It is possible the NAM is for an action that occurred in WWII and was awarded later.
You can request your fathers military records form the National Personnel Records Center in St. Louis.
Link:
http://www.archives.gov/st-louis/mil...out-ompfs.html
Good luck!
G2