Is a son sinful for dealing with his father's house, which was built with a forbidden bank loan?

There is no sin upon you for benefiting from your father's house, which he purchased with usurious loans, because the sin of a usurious loan is related to the liability of the borrower, not to what it was spent on. You did well in advising him. Allah Almighty said: "O you who have believed, upon you is [responsibility for] yourselves. Those who have gone astray will not harm you when you have been guided." And if he proceeded with that, imitating a fatwa from someone he trusts, and not merely seeking dispensations, then he is excused, Allah willing, due to the Prophet's (peace be upon him) saying: "Whoever gives a fatwa without knowledge, its sin is upon the one who issued it." And his saying: "Whoever is given a fatwa that is not sound, its sin is only upon the one who issued it."