Is the eldest son obligated to marry off his elderly father who has no income, given the other brother's refusal and the eldest son's limited income?

If the father is not in need of marriage, you are not obligated to marry him off. However, you must honor him and spend on him in a reasonable manner. But if he is in need of marriage to keep himself chaste, then it is obligatory for you to marry him off if you are well-off, because it is the duty of well-off children to ensure their father's chastity.