Is there any advice that can be offered to a teacher brother to stop accusing his opponents of secularism and apostasy merely for disagreeing with him on matters of Fiqh or politics?

Excommunicating a specific individual is not easy due to the consequences it entails, and it is only applied to those who have met the conditions for excommunication and for whom no impediments exist. If someone declares a Muslim to be an infidel when they do not deserve it, the sin of this statement falls back on the accuser, as stated in the Hadith: "If a man says to his brother, 'O disbeliever,' then one of them has certainly incurred it. If it is as he said, then good; otherwise, it returns upon him." This does not mean that whoever unjustly declares someone an infidel becomes an infidel themselves, but rather the sin of this heinous act falls upon them. However, if an individual fulfills the conditions for excommunication and no impediments exist, then there is no objection to excommunicating them specifically; because excommunication is a divine ruling that must be applied to those who deserve it, and among them are those whose secularism has been established.