*Notice: Currently, you will find that Farkas reverts to steel armor after the Dustman's Cairn quest. I am trying to develop a workaround. Track this mod, and you will be notified as soon as I am able to fix the problem. Ty for your patience!*

UPDATE: The above issue is now solved! Please note that if you have already done the Dustman's Cairn quest "Proving Honor" without this mod, you must use the console on Farkas to re-set his inventory, or he will still wear steel armor. To do this, open the console, click on farkas, type "disable", hit enter, then type "enable" also hitting enter afterward. He should re-appear wearing the correct armor and continue to wear it from now on. A huge thanks to BouncySpartan68 for pointing me to the script that caused Farkas to revert to wearing steel armor after his werewolf transformation during Proving Honor! This mod would not be working without his kind assistance.

What it does:

This very simple mod changes Farkas' default armor to a set of the wolf armor worn by the other Circle members of the Companions. It also fixes a script in the quest "Proving Honor" that causes Farkas' default armor to be set back to a steel set, even after his default character armor has been changed to the wolf set. Why he doesn't wear it by default is beyond me. I would guess it was an oversight by the devs, especially considering the events of your mission with Farkas to Dustman's Cairn. One of the ambushing enemies says "He wears that armor, he dies!", which is in reference to the wolf armor worn by the the werewolf members of the Companions. Farkas was clearly meant to be wearing it, or this comment makes no sense. 

There is a mod that does this for the original Skyrim, but not for Skyrim Special Edition, and the old mod crashes the new game. There is also a SE mod that gives Farkas wolf armor, but it changes his face as well. I decided there was a need for this, so I just whipped up my own mod. Enjoy!