I am sure you know more than a birthplace and date. Getting records from Belarus is likely to be difficult, and will require more information than just a name and date, so it's important to try and find out as much as you can about his later life - those records may give you clues to finding his origins (the town he came from, etc).
He was your step-grandfather, so at some point you know he married your grandmother - where and when did that take place?
Do you know where and when he died?
