He could close the borders if he wanted to--but he won't. He's not even interested in appointing a commissioner who will try to do even a half-@$$ job.
Real border control would mean going against the globalists, the mobs, the triads, the Mexicans, and every other Tom, Dick and Harry who believe that America should be open for rape and the pillaging. If we want real border and immigration control, we're going to have to fight 98% of the politicians.