The entirety of the license is the text of the license. If there is no revocation clause specifying how revocation would happen, the answer is no, it can't be revoked.
Assuming he holds the copyright for the entirety of the program, he could change the license for all future versions, and he also could stop distributing old versions under the old license (putting a new license on it, or simply discontinuing distribution). But, the old license has already granted thousands, or millions, of people, the right to distribute it under those terms.
Assuming he holds the copyright for the entirety of the program, he could change the license for all future versions, and he also could stop distributing old versions under the old license (putting a new license on it, or simply discontinuing distribution). But, the old license has already granted thousands, or millions, of people, the right to distribute it under those terms.