It's confirmation that his father had kept the truth from him; that hurts a little bit, the fact that this was a secret overriding the fact that Noctis knows no parent would want to tell their child they were destined to die. Noctis' own expression is a mix of that hurt and the fact that he's still relieved his father is here. His breadth of forgiveness is apparently much larger for that.

After a moment, he sighs. In that moment he resembles his father much more than he intends to, before he squares his shoulders.

"There's not much I can do. Ardyn will destroy the world if I don't fulfill the prophecy. As much as I don't want to die - I won't live when it'll cost my friends and everyone else their lives. For the time being I'll help here, and then go home to finish it."