He did not fast Ramadan because he was undergoing chemotherapy; what should he do?
Question

I am currently being treated with chemotherapy for liver cancer, which involves daily tablets and intravenous injections. My doctor advised me not to fast, because of the general weakness caused by chemotherapy and the need to drink fluids continually. The treatment will last for six months, then my case will be re-evaluated to find out how successful the treatment has been. It may require two further months of treatment, or using another kind of treatment if there has been no progress, such as radiation therapy or surgical intervention. I hope that you can explain to me what is required of me with regard to this month that I could not fast. If I pray Taraweeh at home, because I am not able to go to the mosque, will the reward of qiyaam (praying at night) be recorded for me? What should I do if I cannot pray qiyaam one night because of being so tired? Should I make up this prayer on the following day?
Praise be to Allah.Firstly: 
We ask Allah, may He be exalted, to heal you and grant you well-being. 
Secondly: 
There is nothing wrong with you not fasting during the month of Ramadan because of sickness, then if you are able to fast after that, you can make up this month, but if you are not able to do so, then you should feed one poor person for each day you did not fast. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 
The one who is not able for it is not obliged to fast, because Allah, may He be exalted, says (interpretation of the meaning): “and whoever is ill or on a journey, the same number (of days which one did not observe Saum (fasts) must be made up) from other days” [al-Baqarah 2:185]. 
But it is clear that inability to fast is divided into two categories: temporary inability and permanent inability. 
Temporary inability refers to cases in which there is the hope that the impediment will be removed, and this is what is mentioned in the question. So the one who is unable to fast should wait until his problem ends, then he should make up the missed fasts, because Allah, may He be exalted, says: “the same number (of days which one did not observe Saum (fasts) must be made up) from other days”. 
Permanent inability refers to cases in which there is no hope that the impediment will be removed… In this case the individual must feed one poor person for each day that he does not fast. 
End quote from ash-Sharh al-Mumti‘ (6/324-325) 
Thirdly: 
The reward of praying qiyaam (Taraweeh) will be recorded for the Muslim whether he prays it in the mosque or at home, although it is preferable to pray it in the mosque. 
If a person persistently prays it in the mosque every year, then he prays it at home because of sickness, Allah will record for him the reward of that in full, as if he prayed it in the mosque. 
It was narrated that Abu Moosa (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “If a person falls sick, or travels, there will be recorded for him a reward like that of what he used to do when he was at home and was healthy.”
Narrated by al-Bukhaari (2996) 
Fourthly: 
If a person misses out on night prayer because of an excuse such as being sick or falling asleep and the like, then it is prescribed for him to make it up during the day. 
It was narrated from ‘Aa’ishah (may Allah be pleased with her) that if the Messenger of Allah (blessings and peace of Allah be upon him) missed a prayer at night due to pain or any other reason, he would pray twelve rak'ahs during the day.
Narrated by Muslim (746) 
An-Nawawi (may Allah have mercy on him) said: 
This indicates that it is mustahabb to persist in reciting awraad (daily portion of dhikr) and that if it is missed, it should be made up. 
End quote from Sharh Saheeh Muslim (6/27) 
So you should make up what you used to pray at night, and add one rak‘ah so that the number will not be odd, because there can be no odd number of rak‘ahs except at night. 
And Allah knows best.