I just realized...if Saitama (from OPM) was way overly cheerful/optimistic (instead of his nihilistic-ish/depressed view) and thought he could learn magic, he might be like our MC here.
Lemme guess. He turns back to a kid and practices magic to obtain mana, but he will be even stronger physically than before and kill more demon lords, and still doesn't have mana in the end.