Its almost entirely guarunteed that Saint-14 is not his biologically (or even adoptive) son in any way. The Exos are Golden Age humans that "gave their body to the ice" in an experiment by Clovis Bray. They are part of Rasputin's Just-in-case-the-Traveler-is-evil plan. For it to be biological, the Speaker would be several centuries old (which surpasses the average human lifespan at this time of about 300). Its very much more likely what you said about Priests being "father" and refering to their disciples as "my son." I, however, have one thing to add: Saint-14 is everything the Speaker wants Guardians to be. -He is a blind knight that does everything asked of him without question -He is a Crusader. Just like our own Crusades where ancient knights went and "cleansed" every non-believer in the name of their God. Saint-14 fights for his cause and will stick to it regardless -He is a Legend. "Follow my instructions and become legend" is what the Speaker wants of his Disciples. Saint-14 serves as what a Guardian fully devoted to the cause of the City would become. He serves as the counterpart to the Speaker's other apprentice, Osiris.

Well we learnt that exos were actually humans with their conscience uploaded into a robotic body (Cayde-6). My best guess is that his son was uploaded into an Exo and became Saint 14 Or they just refer to each other as father and son in respect as priest people do sometimes

Read the legend Saint-14 card. They refer to each other as father and son, no one says it's biological, but student teacher.

I think most if not all guardians would refer to the speaker as this