They tell him that Voldemort is building an army and is attempting to retrieve a "weapon," but is still moving in secret. In this he is actually aided by the Ministry, since Minister Cornelius Fudge is conducting an extensive smear campaign against Harry, Dumbledore, and anyone else who says Voldemort's back. Knowing full well that Voldemort’s return would mean mass panic and then open war, Fudge prefers to listen to his paranoia and believe that Dumbledore is lying and attempting to supplant him as Minister.
A few days later, Arthur Weasley escorts Harry to his expulsion hearing, which Fudge has done everything in his power to slant against him, but testimony from Dumbledore and Mrs. Figg confirms the presence of the Dementors, and Harry is found to have acted in self-defense.
Shortly before returning to Hogwarts, Harry is surprised and a little disappointed when Ron and Hermione, but not him, are made prefects of Gryffindor House.