Grandfather being your father's father, I assume?

Yes, Y DNA is an option but not the first I'd consider.

Y DNA is male only - father to son through the generations. Doing this test can be really good if you're trying to get far back on the male line as Y DNA doesn't change much through the generations.

However, Y DNA tests are far less popular than autosomal DNA tests, which can be used by both women and men. By doing a Y DNA on your brother you are both limiting the pool of matches within other people who have done that test, AND ruling out all the women who have tested too. Yes you might get a match with men whose common ancestor is in the 16th century - but that's not going to help you trace a very recent ancestor, is it?

My advice would be : Get a standard Ancestry DNA kit test. Ancestry is the best choice as they have the largest database and also allow you to download your raw data and upload it to other sites such as Gedmatch. Either you take the test or your brother does - makes no odds. If your father is still alive, would he be up for it?

Look at your matches, try to rule out matches which you think are on your mother's side of the family and not your father's. Try to identify a common ancestor for you and your matches and work forward - if you're matching with second cousins that will be a great grandparent. Rule people in/out - so women children obviously can't father a child, neither can people too old/young etc.

There are lots of FB groups which can help you with this. Another thing to consider is testing someone else you know is descended from your grandfather - a cousin or second cousin. If they are matching with the same people as you are matching with, then you know you have a match on the right side of the family.

It is pretty unlikely his birth wasn't actually registered even if he was illegitimate so it is also worth trying to track down his birth certificate. It may not be where or when you would expect or there may be mistakes in the spelling of his name. Look for him in other places e.g. the 1911 census (if he was born then) which will have his place of birth and age, 1939 register which should have his actual date of birth and things like electoral register could help too. If you can identify a birth record you can send for his birth certificate and see if there's a father named and even if not you will know for sure where he was born which might help with the DNA matching later.

Also agree that his birth will be registered - somewhere. Probably under an entirely different name.

Must’ve been registered in a different place then. Have you tried a general search using the name/date details you already have just not the place?

You also have to keep in mind that people tell fibs. Not necessarily your grandfather but perhaps his parents so he went through his whole life believing for example he was born in Bolton in February when actually it was Bradford in April.

Just keep an open mind and if you can find a birth which is almost right, it might be the correct one.

If he is illegitimate he may have been registered under a different name. Is his surname the same as his mother's or don't you know who she is either?

I'm happy to help if you want to send me a message. I'm nursing a broken right wrist right now and have plenty of time on my hands.

I presume you have tried lots of different spellings of his name, also searching different sources e.g. GRO index, Free BMD, Ancestry etc as it could just be due to a mistranscribed record or a misspelt surname.

Did he have a teenage sister?