What should the eldest son do, who has been forsaken by all his siblings because of his refusal to give his father a sum of money due to his lack of it, despite his previous generosity towards them?

You must be patient with those who sever ties with you, despite your efforts to maintain connection and show kindness to them. You should seek your reward for this from Allah, for Allah will support you against them. It is as if you are feeding them hot ashes with your kindness, and your kindness and patience may eventually end their animosity.