What should we do with a grandfather who refuses to spend on us, even though he is responsible for that?

We advise you to fear Allah, to be gentle with your grandfather, to honor him, to be kind to him, to be patient with him, to be considerate towards him, and to help him fulfill his duty and trust. You can show him the fatwa that clarifies what is incumbent upon him as a guardian and a father, and you can seek the help of some of his trustworthy and advising brothers and friends whom he trusts, so that he may understand your situation and act in accordance with the Sharia.