As will also the circumstances of how our hero's own birth came about.
I think I'm beginning when he's about seven, when his father has re-married with unseemly haste after the death of his mother. People who wish to euphemize it will say that he wants his son to have brothers to his back, but everyone knows that he would rather have a son by his second bride to succeed him.
For one thing, our hero is ignorant of most of his backstory, and has only the faintest inklings of the rest. Putting it in and letting the audience watch him flounder about would create dramatic irony, but I think it would work best if the audience learned it with him.
For another, the story really begins when the stepmother makes a pretense to send him off. Which will backfire badly on her. She would have been wiser to hedge her bets and arrange his marriage to close kin of hers.
Hmmmmm. . . . should his bride, in the end, be close kin of hers? By that point it would do her no good, but it might add something.