How can the zakat be paid on a father's money that has accumulated over many years, given that he does not remember the amount, and is it sufficient to pay a certain percentage of the money now, and do repentance and an accepted Hajj atone for not paying zakat?

The majority of scholars are of the opinion that it is not permissible to delay zakat (definition) beyond its due time. Therefore, your father must repent and seek forgiveness. Among the conditions for his repentance to be accepted is that he pays the zakat for all the past years in which he delayed its payment. If he remembers the number of years and the amount of zakat, he must pay it. If he does not remember, he must take precautions until he is reasonably certain that he has fulfilled his obligation, for zakat is a right of the poor that is not waived except by paying it to them.