So the important thing is does he keep his hero buff when he reverts, or is he powerless until after the slaughter (becoming 17 again). And if he doesn't have the buff yet, he shouldn't have rewind on death yet since that is suspected to be related to his hero powers.
When you really think about it, this story is either going to be one of two things:
1: Classic Hero power fantasy: He keeps his powers even after he went back in time and is now going to kill all the demons and save everyone
2: Depressing Hero story: He doesn’t keep his powers and has to wait 2 years and watch everyone die before killing himself to progress