His king will die.
The others think he's occupied by the petty grievance of his lost eyes, but what he slowly turns around in his head is the inherent conundrum of his vocation.
The paradox is: he was supposed to shape and steer, serve the future king at the expense of the current man (shy boy, sullen teenager, afternoon naps, scattered clothes, rare smiles), for the sake of the kingdom, the world, the oath. This is right and just, but it turned out impossible to serve Noctis and stay out of love.
And so his first failure is: he talks a good game, but in truth he will burn the world to heat up Noctis' breakfast. And so he sits down by his bed and offers (begs) an out, an absolution, an escape.
And so his second failure is: he taught too well, shaped the king too expertly at the man's expense. Noctis refuses.
And so the penance is: his king will die, and he'll walk him to the end of the road. Shape and steer, push and pull, find the right words. Get up and get to it.
Soon, he thinks. Soon. Not just yet. When the train stops.