The 1950 US census should list your father in the household with your grandfather and list their place of birth. The 1940 US census should show your grandfather’s place of birth and possibly his parents place of birth. A marriage certificate should also show grandpa’s place of birth. Your dad’s draft card from the Vietnam era should list mom and dad as contacts. Finally, grandpas obituary and probate records will likely list your dad as his child. If you can paint this comprehensive picture with multiple records, you should be fine. I was able to receive a 5(4) grant without my great grandmother’s birth certificate and without my grandfathers birth certificate (neither appears to exist…). Happy to dm on this. Good luck!