Previously worked for Riot, does not anymore. Which means, unless he randomly gets rehired for this, it's the same as fan art concepts for champions.
He's currently "self-employed." Also, if you care to look at his blog which I dug up, you'll notice he hasn't updated it in 6 months.
And how exactly did this image "leak"? Perhaps this guy was seeking attention, either because he wants to get rehired for the job or he wants everyone to get upset that Riot denied him the chance to sell his work again. --- They called you exotic. Which is just people talk for awesome. Which you are.
You don't have to be employed with Riot to do work for Riot. So no, he doesn't have to be hired at all. --- ill be here 4 u eco jus lyk the mop on the commercial babby-wechina23 fighting games suck-bluerain