He put it out there earlier today on Twitter, where he is @uptomyknees:
I will now offer you an invented Superman Kryptonian backstory that we can all pretend is canon.
Krypton was a planet engulfed in global civil war. A computer system called Brainiac was invented to keep the peace. This backfired. As the warring factions of Krypton united against Brainiac, Mon-Zod and Jor-El, a soldier and a scientist, worked frantically to fight back. They genetically engineered a superpowered Kryptonian warrior, called Doomsday.
The battle between Doomsday and Brainiac was cataclysmic.
In the aftermath on Krypton, a splinter group, “the Daxamites” caught Zod and condemned him to the Phantom Zone, before leaving the planet. As the Kryptonians died out in the planet’s now poison atmosphere, Jor-El blindly sent his infant son out into the universe. This final, desperate act of hope, would ultimately come to forever change the course of history on planet five solar systems away: Earth.