What is the ruling on someone whose father ordered him to deliver zakat al-fitr to a poor person, but he spent the money for his own needs, and how can he atone for that?

Whoever takes zakat to deliver it to a specific person who is entitled to it, must hand it over to them. If they spend it for their own benefit or give it to another person, they are sinful for betraying the trust and are liable for the right until they fulfill it. They must repent to Allah and deliver the zakat to the intended recipient.