Vorstag's class was changed via USLEEP as you've noted to Barbarian. That class has Light Armor as a primary skill and Heavy Armor as an ignored skill meaning no skill points are ever added to that skill so it will never increase beyond it's starting value. His starting LA is 40 and his starting HA is 15, which is what he'll have when you meet him at his minimum level of 10. He maxes out at level 40, where he'll have max level in LA of 100 and still the starting value of 15 in HA. Ok, those are your baseline numbers assuming USLEEP is the only mod you have that edits his character record.
He starts equipped with a Scaled Horn Outfit that includes and Scaled Horn Cuirass with a an armor rating of 32. Again assuming no modded armors from other mods you are using throwing off the power curve for armor protection, the only armors he will ever equip would be a Glass, Stalhrim, or Dragonscale cuirass as only those have a base light armor value rating higher than his default outfit equipment. Base armor rating is important as followers do not account for Smithing improvements when taking into account whether or not an item is "better" for them to wear.
If you still are having issues with him and armor, you might also want to load your full mod list into TES5Edit and see if there are any mod conflicts you aren't aware of when it comes to Vorstag and his skills and perks, some mod may have edited him without your knowledge and is throwing off the tests. Check his skill values in the console in game as well just to make sure nothing is affecting him like a curse or shout -> click on him, then do: getavinfo lightarmor