Have you sent away for his service records? That would provide you with some definite information on his service.http://www.archives.gov/veterans/military-service-records/
National Personnel Records Center
Military Personnel Records, 9700 Page
Avenue, St. Louis, MO
Ask for a copy of everything in your veteran's personnel records ("jackets"). Also ask for the "MED" records. (You must have a serial number and ask for "MED" records specifically, as they are only filed under the serial number and not by name).
On the SF180 or in your letter, you may write (or attach a note): "I am the next of kin. I want these records for information for our family. I would like a copy of all documents in my father's personnel records file. If his records were burned or are missing, I would like to have his personnel file reconstructed. If the record is reconstructed, I would like a copy of all documents used to reconstruct his file and prove his service. The copies of the records I am requesting should include, but should not be limited to the following: Official Military Personnel File, Final Pay Voucher, Medical Records and Clinical Records to include the SGO Tape, Flight Records (for Army Air Corps Personnel only) and Certificate of Service."