The main reason I ask is the whole True King, King of Kings, King of Light etc.
In a sense, it feels like a defiance of the prophecy.
The obvious thought here being that Regis has another son.
Sort've a one child chosen by the Astrals and one by the Kings of Old.
I thought it was an interesting concept, I understand if it's not what you're looking for.
It was just an idea that came to me, that it's Noctis' job to lead the brother on their journey.
If you don't like the idea, I hope someone picks up the prompt and fills it.