It's been decided. Trump cannot block citizens from reading his tweets. District Judge Naomi Buchwald ruled in the summary judgment:
We hold that portions of the @realDonaldTrump account -- the “interactive space” where Twitter users may directly engage with the content of the President’s tweets -- are properly analyzed under the “public forum” doctrines set forth by the Supreme Court, that such space is a designated public forum, and that the blocking of the plaintiffs based on their political speech
constitutes viewpoint discrimination that violates the First Amendment.
If you're a big time Twitter user or have been blocked by Trump you'll want to read the whole thing. (PDF)