If he made the Talbiyah and the niyyah from his home, he can’t change his
mind and must stay in the state of Ihram until his illness is cured.

If before reaching miqat he is overtaken by a sudden illness or accident and has to go home ,does he have to give expiation of a sheep? Although he had not reached miqat but he had put on ihram and done his niyat from home