Dr. Lokin wears Medium Armor, not light. He's very poorly geared when you get him - he doesn't even have a weapon. This means he is pretty much useless to you as a healer when you first obtain him. You'll gear him with the same stuff you'd be wearing if you were a heal-spec Operative.
FYI, if you're a Cybertech or have extra cash/commendations to buy mods with, a cheap way to gear up any of your companions is to buy the level 20 PVP gear from the Fleet and pop new mods in. A few matches a day will get you enough points to buy a decent set of oranges for any of your companions, and then all you have to do is keep their mods up to date.