Is it permissible for a Muslim to shun his brother in Islam if he praises him with non-exaggerated praise, such as saying to him "O Sheikh" because he is his teacher in the Qur'an, or "You are trustworthy to us"?

A believer should not abandon his brother if he praises him with the words "O Sheikh" or "You are trustworthy," because these are expressions of respect and appreciation. The second phrase should be qualified with: "I reckon you to be so, and I do not sanctify anyone before Allah." The dispraised kind of praise is exaggeration or if there is a fear of fitna (temptation/tribulation) for the praised person, due to the Prophet's (peace and blessings be upon him) saying: "You have ruined - or cut the back of - the man." As for praise based on what is known, there is no dispraise in it.