He can’t in New York. If he is a registered practitioner in NYC alone, he won’t be able to practice in any other state either.
For example, I am admitted to practice in a specific state in Aus, but my qualifications allow me to act in any state of Australia provided that I seek leave of the Court/Tribunal to do so which is rarely opposed.
Youre right, doesn’t mean he can’t practice law, but he can only represent himself and he can’t claim his costs if he is successful in litigation if he is acting for himself.