well, we've had some hints, such as the fact that his sprit path/circuit (forget the specific term in this one) was already open but didn't have any power going through it, meaning he awakened long ago, but had his power taken away/sealed. There's also his sisters eyes darkening when transformed(was one at first but is now two after training) which was shown to be a sign of an Impure child with excessively strong energy(though they aren't blood related so that's a factor too). Then there's his absurdly high pain tolerance and the fact that his father was a former Capitan. We've also go no clue on his mother in the slightest. I'll also bring to attention the katana on his left hip that never been mentioned

my guess is he was born an impure child and/or has been exposed to absurdly strong power from birth if not sooner(thanks to his father and sister(also his mother might be a factor)), and has been subconsciously absorbing/using it to the point he awakened at a very young age. His parents believing it was dangerous for him posses such power at that age, sealed it (probably planning to unseal it when he got older, but we saw how that went) and so his power developed under the seal to give him his absurd pain tolerance, but since its sealed that's all it could do.
the katana is probably his personal soul weapon unlike the sword he got from Nue. so when he eventually unseals his own power he'll use that katana instead of or along with Nues sword.

As for the second point, I guess, technically. Hope they raise it right.

Behind the curtain are 3 of the 4 resurrected hyos