Armor: Light armor, medium armor, shields (druids will not wear armor or use shields made of metal)
That isn't fluff. Those are the rules.
Unless you consider your armor proficiency fluff.
The reason may be fluff, but the rules say that he will not wear it. Which means he will not put it on. So it doesn't matter if he is technically proficient, because it will never adorn his body. Full stop.