Is it permissible for a patient suffering from salts and kidney stones, who experiences dehydration, to fast Ramadan and then break his fast, and is it permissible for him to feed the poor (as expiation)?

If fasting increases illness, prolongs it, or causes hardship to the afflicted, then one may break their fast and make up for it after recovery, as Allah Almighty says: ﴿But whoever among you is ill or on a journey [during them] - then [he must make up] an equal number of other days.﴾ (Quran 2:185). One does not move from making up the fast to feeding the poor unless the illness is chronic and incurable; in such a case, one feeds a poor person for each day. It is best to consult a doctor to ascertain the extent of fasting's impact on the illness. However, breaking the fast due to illness is not exclusively tied to a doctor's advice. If fasting causes unbearable hardship, or if one knows from experience that fasting is difficult or delays recovery, then they may break their fast even if a doctor has not advised it.