User blog comment:Nobody700/How is he so strong?/@comment-27506972-20160201131907

Well I don't want to spoil the webcomic to others, but it was sort of hinted how he got his power. I think it's because he broke his limit from not stopping his training even though he had a frail body. It's different from other heroes that train hard, because they don't train beyond what they are capable of. Like how a really fit athlete can do Saitama's workout relatively easily, but that won't push his limits like it did for Saitama.