If you follow his little questline you can eventually get your hands on the robes, they aren't great but nice for collecting!
Copied from the wiki page:
Worn by Ildari Sarothril in Highpoint Tower. They can only be obtained at the end of the quest "Old Friends".
Master Neloth wears similar, yet more powerful set that is unobtainable. His robes have +150% Magicka regeneration, and a value of 175 Gold. Weight and appearance are identical to the obtainable version. (xx01cda8)
The Telvani robes you get at the end of his questline are weaker versions. What I mean is that the enchantment is not as good. Try using pickpocket on him
Wait, So if I give him Fur armor, He'd eventually wear it? Or must I give him the Archmage Robes in trade? Tell me how to get his stuff without cheating!