I assume there is more about his backstory that he hasn't been open about yet.
Well, an important one that people seem to be forgetting was that we did get to see a small image of his past before, which was his older brother (presumably the bad guy under helmet in this chapter) telling him that at least one of them had to get out and see the world. Honestly, that's enough already to infer A LOT about his past and that it was not good at all. Really, the only thing we don't know is how exactly he ended up in the dragon in the first place. You don't seem to end up in the stomach by accident.