You can’t get a declaratory judgment from a federal court without an actual case in controversy.
Theoretical controversy, such as that debated by scholars and pundits in the political arena, does not qualify as a legal controversy.
Just saying, you are right, he can't sue to get something that he already "holds," even if it is improvidently granted or held. And it's nonsense to sue (the government) to have it taken away!
He has to play this by the adversarial rules, including all procedural challenges. Out legal system is adversarial, and he has a right to defend his claim.