Do you have your grandfather's death cert? Where does that say he was born?
Do you have his parents' marriage cert? Where were they married?
Was he born near the date of the marriage? Could be near to where he was born.
Have you checked free bmd?
Is his name a common name? Are you able to whittle the list down from bmd?
When I find an entry I often email the register office asking if the birth date matches my person and if the parents are ..... (name)
Hope this helps.