The any solid metal would work but I'd recommend a different power he may be at a disadvantage against a few NPC's. A power similar to a freedom theme that's a good question. Could be an irony thing with chains perhaps since he was constricted in his life, for freedom perhaps wind or something? Anyway avoid Ethereal, so long as it's not too powerful most magic would be okay even elemental.

Edit:Pathfinder you are good having someone with that personality and backstory will be useful.