If you have found the birth registration for your grandfather or have obrained a copy of his birth certificate, you will have the details of his parents and thier address, occupations and marriage date. This means that you know the information that needs to be on any certificate for any other siblings. by eliminating possibles you should be able to narrow it down, i think you can look at the original certs. on film , and i am sure the registrations are indexed like England and Wales, based on the registration district , you shoulod be able to pick possibles for checking the actual register, not 100% sure on the Scottish research aspect, but certainly that would work. It's possible to do this partially online but this site will be useful
http://www.scotlandspeople.gov.uk/
In England I would send to the resiter office an order for an entry but ask that it only be sent if the details match the known facts.
Someone with experience of The Scottish registration access and availability should be able to advise further.