I still don't buy it, for several reasons.
The first is that his eyes are untouched, and he can clearly move his mouth. That means he can have a reaction to something Koh does; he can still emote, and thus can still have his face stolen.
The second is, what possible motivation could Koh have for helping this guy? Koh's creepy as hell, but he helped the Avatar the last time the Avatar found him, and Koh displayed no proclivity toward destroying the Avatar or anti-bender sentiment.
As such, it might as well be any spirit. I don't believe Amon's claims, but even if I did, there's no evidence to suggest Koh's involvement over any other spirit.
I think we'll quite likely see Koh again, but I don't think it'll be as Amon's mysterious spirit ally.