Ignis can't see Prompto's face, but he can almost hear the click in the blonde's mind as he puts two and two together. He hears it so strongly, that he can tell what the gunner is going to ask before the question finishes leaving his mouth.

"Yes, he knew."

Regis Lucis Caelum CXIII. The Father. His moniker among the Lucii. This Ignis learned when he put on the ring and faced the Kings of Old in Altissia. The title is the only one that could ever be given to the man who fathered the Chosen King of Light and had to bear the pain of knowing that his beloved son would never rule the kingdom he sacrificed so much to protect.

The kingdom he sacrificed being a proper father to protect.