Is the general discourse of the Messenger (ﷺ) caprice, or does it have another classification, and what is the way to establish proof against one who considers it caprice?

The Almighty's saying, "Nor does he speak from [his own] inclination," means that the Quran is not from the words or inclination of the Prophet (peace be upon him), but rather it is a revelation from Allah. This does not mean that every word spoken by the Prophet (peace be upon him) is a revelation. He may exercise personal judgment in worldly matters or legal rulings. If his judgment is correct, Allah affirms it, and if he errs, Allah clarifies it for him. Thus, everything that emanates from him (peace be upon him) is truth, and it is obligatory to believe him, follow him, and obey his command.