Re-reading that, a 15 yo travel back in time to have sex with his grandmother so she can give birth to a son the will have a son and this son will be old enough in 2024 to travel back in time in 2009 and impregnate his grandmother ? He biologically couldn't originally come from 2024, his father hasn't had time to develop the ability to reproduce.

Or anyone is time travelling and your explanation is basically just magic.

Definitely Alabama vibe though :].