Is a young man at fault for abandoning his elderly father, who lives alone in a rented apartment with an indefinite lease, believing it is forbidden to benefit from such a contract, and what should he do regarding his father?

An indefinite lease with no specified duration is not Islamically valid, even if permitted by law. The father must rectify it by setting a duration for the lease. The son should advise his father regarding this. As for him leaving his father's house and letting him live alone, this needs clarification: If his leaving entails neglecting his father and his father needs his care, then it is not permissible for him to do so, because the harm of neglecting and disobeying the father is greater than the benefit of leaving the house. If his leaving does not entail neglecting his father, then there is no blame upon him for leaving the house, and he will be rewarded for forbidding evil. In all cases, he must strive to convince his father of the invalidity of this lease and seek the help of scholars in doing so.