What is the ruling on laughing at and ridiculing a muezzin who tries to imitate the Adhan of the Grand Mosque in Mecca?

Not all laughter or mockery constitutes disbelief (kufr). If your brother did not intend to mock the Athan (call to prayer), he does not become a disbeliever. However, if he intended to mock the Muezzin (person who recites the Athan), then this is impermissible, and he must repent. If he did not intend to mock anything, then there is no sin upon him.