If Nyx knew Prompto better perhaps he would have pressed but they knew little of one another. Nyx knew his name and his face, his friendship to Noctis and the other two. But nothing beyond that.

"People are allowed to need their friends without being accused of being selfish."

Nyx did his best to not need anyone but it didn't mean that the presence of the familiar wasn't a comfort.

"Temporary separation is still just temporary. You'll have many years yet to enjoy after the war is finished." If they both survived it anyway. Better to be optimistic in this case.