Birth Certificate
zaslal Frank (confidential) v Fórum Nemecko
Only direct line relatives can get a birth certificate, therefore nobody will be able to help you - you’ll have to apply for it by yourself (showing proof that he is your direct grandfather).
You need to contact the family registry (Standesamt) of the town where he was born. If you don’t know where, you must find it out - there is no central register of births in Germany!