What should I do regarding my father, who is ill with kidney failure and poverty, knowing that he used to steal?

You should advise your father to repent to Allah. Repentance is achieved by abstaining from theft, resolving never to repeat it, feeling remorse, and returning the stolen goods to their owners if they are known, without informing them of the theft. If he is unable to return them, then he should seek their forgiveness or frequently seek Allah’s forgiveness and supplicate for them. If the stolen money belongs to unknown individuals, then he must give it as charity to the poor and needy, and the reward of this charity will be for the owners of the money.