A person has two sons who are chronically ill. They have never fasted before. What is this person (the father) obliged to do regarding them? And how should the two sons perform ṣalāh?
Fasting is not obligatory on a person who is chronically ill because of his inability to do so. He does however have to feed the poor person in place of each day missed if he is sane and past the age of puberty.
There are two ways to do this: firstly he can prepare lunch or dinner and then invite the number of poor people he needs to feed according to the number of days he did not fast, just as Anas ibn Mālik would do in his old age. The second way is that he can distribute raw food and the poor person receiving it cooks it himself. The amount of this food should be a mudd of wheat or rice and this is calculated according to the sāʿa of the Prophet which equates to four mudds. A sāʿa weighs 2kg and 40g which means a mudd weighs 510 grams. This is the amount of rice or wheat a person would have to feed and he should add some meat to that. He should also apply this to previous fasts missed.
Regarding the performing of ṣalāh, the two sons should pray however they are able as the Prophet said to ʿImrān b. Ḥussain ‘Pray standing, if you cannot then sitting, if you cannot then on your side.’
Majmūʿ Durūs Wa Fatāwā Al-Ḥaram Al-Makkī, Muḥammad b. Ṣaliḥ Al- ʿUthaymīn, Volume 3, Page 51