We tried to get retired military ticket for my dad, who is a vet but never voluntarily joined the military. They said No way, Jose!
...and they thanked him for his "valiant" service.
If you Dad actually Retired from the Military , 20 year career with a Government pension than he would be eligible for the discount . Your dad is probably a Vet like me..a Few years of military service and out with a Honorable Discharge,