So I picked up Mercurio, a mercenary mage, at the inn in Riften. We were on a quest with a lot of loot, so I had him carry some things for me. This just happened to include a full set of steel armor. So he decided to equip it, as companions often do when you hand them armor. But he won't uneuip it; I get error message "You can't take your teammate's starting armor." Obviously, this wasn't his starting armor. His starting armor was the Adept Robe of Destruction.
So now I have an utterly useless mage running around in full Steel armor, shield and everything, and apparently it's somehow become his default armor. I would really like to fix this, as he was quite the useful companion.
How do I solve this?