Can someone remind me who was the old man?

He's the man who jae shin gambled in the beginning of the story. Jaeshin killed his son in exchange of prolonging his life.

The old man’s life?

yes I think jaeshin didn't tell the old man that his son would be sacrificed for prolonging the old man's life. It was already too late and the son died, so the old man has basically been living in regret and wanted to hurt jaeshin back by doing this. It was hinted several times throughout the novel.