Is the repentance of a father who took out an interest-bearing loan valid, and does he fall under the Hadith: "His food is unlawful, and his drink is unlawful..."? Is the house built with money from the loan considered unlawful property, and what should be done about it? And will the supplication of the son for his father be answered in this case?

Whoever repents, Allah accepts his repentance. Your father has done well in repenting from usury. The prohibition of a usurious loan is related to his responsibility, so there is no prohibition in benefiting from the house. He must be sincere in his repentance, regret, seek forgiveness, resolve not to return to it, and hasten to repay the loan if he is exempted from the interest. Your supplication for your father is a legitimate act of filial piety.