Is it obligatory to repeat the prayers a person led as an Imam while in a state of major ritual impurity (janabah), knowing that he was ignorant of the ruling and his father had commanded him to do so?

Whoever prays without purity intentionally has committed a grave sin that necessitates repentance and making up the missed prayers; because one's obligation is not absolved without them. Repentance is also required for the ignorance of learning the obligatory knowledge. As for the prayer of the congregants, it is valid.