Finding father?

[SumRum]

My grandfather grew up thinking that one man, (his mother's husband), was his father,
however, before his "dads" death, he was told by him that he was not his real father.
The only thing about his real father that is known for certain, is his surname.
Through which ways should I go about finding his father's name?

[Lisajj]

Your grandfathers birth certificate?

[avm228]

Hi and welcome

Unfortunately the birth certificate is unlikely to solve your conundrum if the "false" father was married to the mother at the time of the birth.  In those circumstances he would be the lawful father of the child (and properly registered as such) even if he was not the natural father.  So a finding that he was named as the father would not prove whether he was or was not the natural father.

So: were they married to one another at the time of the birth?

[SumRum]

Yes, they were married at the time.
However, he was away at war during the time of conception.

[avm228]

Yes, I see. So he would have known for sure at the time that he was not the father.

You may as well get the birth certificate if you have not already got it to see whether anything useful appears.  However, as I say if it shows the mother's husband as the father that isn't proof that he was (biologically speaking).

Have you looked for a baptism? Some clergy included information which could be helpful in tracing fathers of children born outside wedlock, though this was much more common in earlier times when the parish was responsible for such children.

Are there any elderly living relatives who might have relevant information?

[Lisajj]

Unless the natural fathers name appears on the birth cert or in the baptism records, then I can't see how you will find out the truth.
I have had the same problem with my great grandfather - his father was not named on the birth cert or in the baptism records.

[Redroger]

I think the only possible route is by a Y chromosome DNA test, and even then it is unlikely to be conclusive unless the immediate family members have been tested.

[pinefamily]

I don't suppose your grandfther had an unusual middle name, eg John Turner Smith? That could lead to a possible clue.
When you say you know the surname, how do you know this?

[Ruskie]

If you know the surname of the real father, if it is unusual you might be able to find him living near your grandmother in the census - perhaps 1911 or earlier depending on which war you are talking about.