Awkwardly enough that does sound like something John would've done as his former self, he once threatens to make [human protag] his cyclically flayable skinless pet for all eternity. Cringe behavior tbh. If he ever gets to hear Apophis's backstory it might inspire him to feel freshly weird and guilty about his past, which is delicious to me.

OH AND John can sense an aura of light or darkness around people that are possessed by particularly powerful eldritch forces. Would he get a darkness vibe from Apophis? That would be a fun early clue, tho John is from too early a canonpoint to have many points of comparison, so he'll just be nebulously suspicious and wary.