He hasn't said much in the way of specifics. It...sounds like he thinks he's from right after Altissia fell and Lunafreya died, but I can't figure out when he might actually have come here from. He's far younger than he should be for either myself or Noctis, so the absolute latest that could potentially place him to my knowledge would be after-

[...]

...after the sun stopped rising for a decade or so.