Under usual process of research, you have already started by getting documentation from yourself back to gr grandfather.. at least that is the normal 'start'. When you make the statement that the grandfather is from Spain, normally you would be making that statement from some document or record, which verifies what is stated. What is that document? It should have some specifics for you to work with.. or suggest other records of value.
However.. if you are making the statement based on family knowledge, you have not been collecting records to confirm the information. The point of using documentation is not just to 'prove', but also to give further info.
Example.. have you located him in census records? I don't know if you are in the US.. if you are, that should indicate a way to estimate any immigration, and using that date, look for that record.