Suprising Large DNA match

[Bhx7]

Hi

I have had my DNA done through Ancestry for sometime and have had some quite good results which has helped me prove my previous research and extended it considerably further in places.

However, earlier this year I had a large DNA match of 520cM with someone out of the blue. From the story he was told he had always known his mother was not his grandfathers daughter, but her father was in fact a member of my family who owned a shop. It was close, the location of the shop was actually next to the Public House my Gt Grandparents had. So close enough.
His matching links to other members on Ancestry and also other forums, also prove beyond a shadow of a doubt that he definitely links to my paternal family. But how???

As his mother and grandmother are no longer around, there is no one to ask regarding who in my family line it was who is his biological grandfather. There are however, 4 possible candidates. My Gt Gt Grandfather, Gt Grandfather or 2 of my Gt Grandfathers brothers

With the link being so close I had assumed he would be my Gt Grandfathers grandson. However, my Gt Grandfather would have only been 15 and his grandmother would have benn 24! Not unheard of I know but a little concerning.

I have tried DNA Painter and no matter which way I try, I can't seem to come to a satisfactory conclusion.

However, there now seem to be a complete lack of my Gt Gt Grandmothers family DNA within his matches, which if he had been my Gt grandfathers or the other two brothers, surely he should have had?

So my Gt Gt Grandfather seems the most likely, but the DNA match is just too high, isn't it?

I hope all that makes sense.
Any insights would be gratefully recieved.

Thank you all in advance
Brian