No. There is no domestic terrorism law. That's why domestic terrorist groups operating within our borders are declared "criminal enterprises." No law allows you to legally define a group within the US a terror group. We can only declare foreign groups, not on US soil, terrorist groups. He might be able to approach it from the perspective that since Antifa is a global group, they're a global terror group. We'll see what happens. But this was the roadblock.

EDIT: And as far as I know, Congress isn't needed for groups to be added to the US list of terror groups. I think it falls under US Dept State, but maybe someone else can chime in.