How did the man who conversed with his companion know that the creation of man passes through the stages of dust, then a drop of fluid, then being fashioned into a man, before the revelation of the Holy Quran?

The extent mentioned regarding the stages of human creation in Surat al-Kahf is not from the hidden knowledge. Rather, it is among the fundamental principles of common sciences and knowledge about which rational people do not dispute, such as the origin of man being from dust, and Allah made that among the signs indicating His power. The disbelievers of Quraish did not dispute this; rather, it was an accepted fact for them, as in His (Allah’s) saying: "And you have already known the first creation, so why do you not remember?" and His saying: "O mankind, if you should be in doubt about the Resurrection, then [consider that] indeed, We created you from dust." And even in the argument with the Christians concerning the creation of Jesus, they did not dispute that Adam was created from dust. As for the detailed stages of embryonic creation, which were not known until after the revelation, they are mentioned in other surahs such as Al-Mu'minun and Al-Hajj. Surat al-Kahf did not address these details; rather, it confined itself to what was known to everyone, and that is from the etiquette of debate.