Hi Andre,
if memory serves, Frederick Thorne turns out to be one of the aliases Quentin had adopted over his century of perpetual youthfulness to forestall what might be called the "Dorian Gray syndrome." I think Thorn was Quentin's persona during the 1940s. There's an episode if you keep watching where Julia finds something, maybe a driver's license, that Q had when he was Thorne.
I just love the early Leviathan storyline.
G.