I believe the VA may require a copy of your Father's separation document to verify his service. To obtain a copy of the separation document, you may download and complete a Standard Form 180 (https://www.archives.gov/files/sf180-request-pertaining-to-military-records-exp-april2018-1.pdf ).
On the Standard Form 180, please indicate you are also requesting information concerning veteran's benefits.
You may then mail the completed form to the National Personnel Records Center at the following address:
National Personnel Records Center
1 Archives Drive
St. Louis, MO 63138
You may also fax the completed form to 314-801-9195