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

No File Exists
Check below answers in case you are looking for other related questions:
The pregnant woman is not prevented from praying and entering the mosque
Making up Ramadaan fasts missed because of childbirth

No File Exists
Related Answers:
Exams are not an excuse for not fasting in Ramadaan
If a pregnant woman feels a bit tired can she break her fast?
Breaking the fast in Ramadaan because of exams
The pregnant woman is not prevented from praying and entering the mosque
Making up Ramadaan fasts missed because of childbirth

Recommended answers for you:
When should a person make the intention to fast, and what if he finds out during the day that Ramadan has begun?
Days on which it is prescribed to observe naafil (supererogatory) fasts
The reason why it is forbidden to sleep on one’s belly
The benefits of drinking camel urine
Conditions of wiping over socks