Can one absolve oneself of a remaining debt to a driver, if it is impossible to reach him to give him the money directly, by giving the amount to his son without informing him of the story, or by transferring the money to his account without informing him of the story?

This money must be given to this man, even if by stratagem and without his knowledge. Inquire about him through his son. If the father is alive, the money should be given to the son to deliver to his father. If he is deceased, the money should be divided among his heirs. Your husband can tell the son that this money is a debt owed to his father. The important thing is that the money reaches the man if he is alive, or his heirs if he is deceased.