I think his dad was either Head of IT, or the shadow to Head of IT. There's no way anyone gets in the vault except those two. We've seen that in our Silo. I think his dad got scared because the rebellion was overtaking the silo, and he feared for his son's life, so he put him in the Vault, gave him the code, and told him "don't let anyone in". That's why Solo kept repeating that phrase a couple eps ago.
I don't think he has any other big secret other than that, because he was probably a kid when it happened. I have no idea what his real name is, or if there is/was even an actual Solo, or he made that up.