Responses
I think that Zeno know him remember that Zeno knew all the other 3 dragons when they were kids and also in one chapter when he was with the withe dragon as a kid he saw the sky and said that yona was born. Maybe he went to the castle to see and he knew so-woon there, omg maybe Zeno know the true about the murder of so-woon father (my own thinking).
I don't remember where exactly, but Zeno said Soo Won gave off a strong similar vibe with Kouka's first king. And perhaps yes, he might have visited Soo Won when he was a kid.
I'm really curious now if Zeno has any other relationship to Soo Won. There's this reaction here:
http://www.mangago.zone/read-manga/akatsuki_no_yona/an/v15/c088/pg-7/
when they first meet and I feel like it's beyond Zeno recognizing who he is. And then there was this current chapter. I just feel like there's something more. Though I don't think Soo Won knows him at all.