Yeah, someone's being unreasonable here.
It's either
(1) The guy who is putting in the work to run the game, and is working to run a game that he and the other players will enjoy, OR
(2) The guy who wants to DM, but can't find any players willing to play if he does so, and who has attempted to usurp the first guy's authority over the game he's running.
Hrm.
I can see why some people find this a tough call.
Why's it either one of the other? I can't see any reason why we can't declare them both unreasonable.
That's pretty much my take on the situation. Of course, I doubt we really have the full context to properly judge things, but from how it's been described... we've got one player who isn't really the best addition to the group, sure, and is being unreasonable when he tries to circumvent the DM's restrictions on character options. At the same time, we've got a DM who is being unreasonable when he forces someone to play a character they don't want to play.
I'd say both have room to compromise.
Now, that doesn't mean the troublesome player will be willing to do so! But in this case, the DM is the one coming to us for advice.
And my advice remains: Try and work with the player while setting appropriate boundaries. Explain why you don't want him using this specific new character, and work with him to see if you can figure out some character ideas that he likes and that will fit in the game. And give him the opportunity to try and play a character he will be happier with.
You don't want to give him the impression that he
can get away with whatever he wants. But you do want to give him the opportunity to enjoy the game.
Now, if this doesn't do the job? If it becomes a recurring problem, he wants to keep changing characters, or he isn't happy unless he is always outshining another player? Than you have an issue. But the most likely solution isn't to force him to do what you say - honestly, it is to have an open discussion with him and see if he is willing to work on those problem areas. And, if not, to ask him to leave the game.
Letting him keep playing while forcing him into an uncomfortable position is only going to keep building those tensions and causing problems within the group. And that isn't really a winning solution for
anyone.