Ultimate Optimizer's Multiclassing Guide

[Arkhios]

Right now he's using heavy (chainmail), but I plan on switching to medium (half-plate), which works out to the same AC but will allow him to rage.

Then you're good.

Although, a minor nitpick: it's not that heavy armor wouldn't allow you to rage, it's just that rage doesn't function as well in heavy armor as it does in medium armor or unarmored. But, as I said, it's a minor detail. Still, a detail worth mentioning, because if you read the Rules as they are written, some Barbarian sub-class features keep functioning normally in rage, regardless of the type of armor you use. Its the baseline rage that gets wonky with heavy armor.