As a GM ,I use the following method..
When starting a campaign or introducing a new player, I set the rules on what is allowed, and do not, under any circumstances, back down from it. If you don't want Kitsune in your campaign for any reason, explain the reason and don't back down. As a GM, you have that right.
If you allow him to play a Kitsune with the intention of having a mob later capture him and pick him apart for study or purging him with fire for being a demon, that, in his eyes, is just plain mean.
Also, if you let him have something out of place, soon, other players will want out of place stuff too, and you won't be able to tell them no for reasons of apparent favoritism and the good old "why can he have it and I can't?".
Nip it all in the bud and tell him, sorry, no Kitsune in this setting. Deal with it.