What is the ruling on addressing a colleague with the title "Amir al-Mu'minin" (Commander of the Faithful) after one of the professors gave it to him and it became common among students?

There is nothing objectionable in calling a student who has memorized the Quran "Amir al-Mu'minin" (Commander of the Faithful) if the intention is to acknowledge his scholarly excellence and memorization of the Quran, and not his leadership of a group or rebellion against authority. This title was prevalent in ancient times among learned memorizers, and none of them rejected it. Examples include Al-Suyuti and other scholars who referred to imams of Hadith (definition) with this honorable title, such as Shu'bah ibn al-Hajjaj, Sufyan al-Thawri, Ahmad ibn Hanbal, Al-Bukhari, and others.