What should we do about a grandfather who does not pray and offers illness as an excuse, despite his ability to perform strenuous tasks?

Illness is not an excuse to abandon prayer. The grandfather must be advised and reminded of God gently and wisely. If he insists on abandoning it, he should be threatened with reporting him to the competent authorities. If he continues to refuse, his case can be elevated to the Sharia court to compel him to pray. Ibn Uthaymeen and Ibn Baz issued fatwas that it is obligatory to report the one who abandons prayer to the ruler or the Sharia court to compel him, citing the Almighty's saying: "But if they repent, establish prayer, and give zakat, then let them go their way."