Is what the questioner said—that Allah will cause a person's good deeds to be swallowed up because of his support for oppressors—considered an error, or does it amount to disbelief? And can the questioner's own good deeds be swallowed up because of this statement? And what is the repentance for that?

What you said is an improper statement, due to the fear that it might be speaking about Allah without knowledge. The correct thing to say is that whoever dies upon this is in danger or exposed to Allah's punishment. Explain to your father that Allah is a Just Judge who wrongs no one, and remind him of the warning against those who aid oppressors, so that he may find comfort and reassurance in Allah's justice. Applying a threat (of punishment) to a specific person is not appropriate, and that is not disbelief (kufr). What is obligatory upon you is to repent to Allah and not to issue judgments on specific individuals, because ultimate outcomes are hidden and the door of repentance is open.