Proud boys are an organization. Antifa is a movement. Unfortunately this won't go anywhere. Even if he could make them a terrorist organization, that will just feed into it, and make them feel that what they are doing is worth it.
A movement? Have you ever looked into the history of Antifa? Are you sure you don't need a definition? Why are you so reluctant to accept some facts and so eager to accept others? Yes. Antifa is an organization that likes to call themselves a movement. It's more inspiring to call themselves a movement.
What this would do that would be of use is avail the FBI of some tactics of investigation that would not be available to them otherwise. Maybe it makes the beta bros feel important. But the really important thing is that it makes it easier for law enforcement to catch them committing actual crimes.