I'm already seeing so many problems with the writing. He sad it had been five years since he returned to Earth. In the other world it has been 180 since he slew the demon king. So there's a x36 time gap. If it took one month to return to Earth, that means it took 3 years to kill the demon king.
Was his age reverted along with his abilities? Why was there no mention of how people reacted to a whole classroom of kids just vanishing, and then a month later a single one returns out of thin air?
If all if his abilities were reverted, how did he kill 1,458 demons in a single month? That's 48.6 kills per day
What has he been doing for clothing or shelter or food?
Also, why did the god have to go searching for him? Were they not the one who originally summoned/returned the kids?