Who was the dying kid in hospital

I have no idea and I never went to the bathroom, while the movie was on.

Earlier in the movie he went to the kid's birthday party with his ex-wife and has a conversation with the father. We find out he was a cancer patient that was friends with his dead son.

This is one aspect of the film that needed some improvement.

I knew the dying kid was the friend's child (though I could see how a viewer could easily miss that detail). However, the big problem is that the audience doesn't get Affleck's attachment to this kid. Why, exactly, is Affleck so upset that he runs out of the hospital and drinks hard? Was he so close to that kid and his family? Or did he just get memories of his kid's death by seeing that family grieve?

They needed to flesh out the dying kid in the hospital.