What if the son is unable to find a job that provides him with an income guaranteeing the fulfillment of the condition of having to pay back the usurious student loan?

If the loan was usurious and the borrower was unaware of its prohibition, there is no sin upon him for what has passed. After becoming aware, he must cease new usurious dealings and terminate ongoing ones as much as possible. This is based on Allah Almighty's saying: (And there is no blame upon you for that in which you have erred) and (So whoever receives an admonition from his Lord and desists, he may have what has passed). If the borrower finds work that provides income to repay the loan, he must repay it. It is preferable to repay only the principal amount and forgo the usurious increment. If repayment is not possible except with the usurious increment, there is no sin upon him in that repayment as long as he borrowed it before becoming aware of the prohibition. If he does not find work that allows for repayment, there is no blame upon him, for Allah does not burden a soul beyond its capacity.