Narrowing down which brother is the father

[TickleMyPickle]

My Dad (77) is adopted and several years ago we were fortunate enough to be able to find his birth mum( sadly deceased) and half siblings.
The siblings welcomed him with open arms and despite living on the other side of the world, they have all met several times and have forged a wonderful relationship and have been able to give him photos and information about his mum.
There was no information at all about his birth dad, but he did a DNA test and we uploaded it to the Ancestry website and he had a match for a first cousin on his dads side.
The cousins mum had no male siblings so we know that it must be a one of his Dads brothers….of which there are 7 ( one died before my Dad was conceived, so 6 that could be his Father)
This is where we draw a blank, how can we try to narrow down which brother it is?
I’m a complete novice with all of this, but would dearly love to be able to solve this for my Dad.
I’m assuming none of the brothers descendants have done a DNA test or it would’ve already linked them, so is there anyway of working it out or narrowing it down?

DNA wont tell you if their decendents haven't tested so.you'll have to do it the oldfashioned way. Can you work out from census records, work documents, passenger lists, marriage certs etc where any of the brothers were in the relevant time frame?

you have 6 people to possible contact to enquire delicately if they knew your grandmother. You would need it to be a sensitive letter and hope for the best at this point. It could be the boy/now man didn't know he became a father?

I would be happy to do some research around the brothers for you if you want to PM me. However it would only be as NoBinturongsHereMate says - to try and see if anyone was placed in the right place at the right time. That may be meaningless if they were all there at the right time IYSWIM.

"................the cousin's Mum had no male siblings......."

That she knows of.
Are you sure you can write her side off ?