Fight meta with meta.
Point out what's happening off game. Try to see if he actually has a good reason for it.
If he does, adjust as needed.
If he doesn't, or won't give you a straight answer, kill the character 1st chance you get. The character behaving like that is pretty much under some sort of demonic possession (ie. Player) anyhow.
(Generally speaking, trying to deal with "unreasonable" meta in-game is a waste of time. It isn't always unreasonable though, so the point is to confirm whether it is or isn't instead of trying to get the person to go your way or whatever.)