It's never stated definitively, but since he can wear the Amulet of Kings, which is otherwise impossible unless you're Dragonborn, and following some lines in the Commentaries, such as:
"Offering myself to that daybreak allowed the girdle of grace to contain me. When my voice returned, it spoke with another tongue. After three nights I could speak fire."
Ah ok. Does sound interesting that quote and what it may imply. Guess Dagon may have helped him there perhaps.
@ OP: Also keep in mind from earth mythology, many demigods came into conflict with the gods, such as Heracles and Achilles. (Don't know if it's completely accurate to call Dragonborn demigods, but similar I guess.)