Can I give zakat on the money my father lent me, and must I convince him to give zakat on the rest of his savings accumulated over 30 years?

Zakat is a great pillar of Islam, and whoever withholds it is sinful. Whoever denies its obligation is a disbeliever. And whoever acknowledges its obligation but refrains from paying it is in grave danger. It is incumbent upon the Muslim ruler to take it from him, even by force. No one other than the ruler is permitted to disburse zakat on behalf of another.

The questioner must advise his father, supplicate for him, and seek assistance from those who can influence him.

Zakat remains an obligation upon the father, and he must pay it for the entire period that has passed. If the father dies before paying what is due from him, it must be disbursed from his estate before its division.