In this instance the birth certificate is a primary source and all the other sources are secondary.
I.E. The entry in the birth register was completed just after the birth.
The other sources are primary sources for other events death certificate (primary for age/death, secondary for birth), marriage certificate (primary for marriage, secondary for age/birth) etc.
If your grandfather had siblings their birth certificates may confirm the birth date (i.e. if a sibling was born a few months before or after your grandfather's date of birth then doubts would be raised).
Cheers
Guy