It seems Reiji's mother saved Esemori when he tried to commit suicide. Later they apparently lived together, but for some reason their relationship went wrong. Also, Reiji probably isn't Esemori's son, looking at that cut-offed line, assuming it is Esemori is the one hitting him with an ashtray. There's also another option that Esemori is Reiji's father and it's another man who lived with them and hit him, but I think that's less likely.