He is sick and the doctor told him not to fast
Fasting is obligatory for every adult Muslim who is able to fast. If a Muslim is unable to fast because of a sickness and he will be harmed or the sickness will be made worse by fasting, or if he needs medical treatment during the day in Ramadaan such as pills and medicines and other things that are eaten and drunk, then in his case it is prescribed not to fast, because Allaah says (interpretation of the meaning):
“and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days. Allaah intends for you ease, and He does not want to make things difficult for you”
And the Prophet (peace and blessings of Allaah be upon him) said: ‘Allaah likes you to avail yourselves of His concessions as He hates you to disobey Him.” Narrated by Imam Ahmad (5839) and classed as saheeh by al-Albaani in Irwa’ al-Ghaleel (564).
For more information please see the answer to question no. 11107
Based on this, if it is proven that fasting will harm you, based on the testimony of a trustworthy doctor, then you must not fast and it is not permissible for you to fast, because Allaah says (interpretation of the meaning):
“and do not throw yourselves into destruction”
And the Prophet (peace and blessings of Allaah be upon him) said: “There should be neither harming nor reciprocating harm.” Narrated by Ibn Majaah (2341) and classed as saheeh by al-Albaani in Saheeh Sunan Ibn Majaah.
Because you fasted last Ramadaan and were not harmed by it, what we think is that that you should consult another doctor who is more trustworthy and more knowledgeable than the first about your condition. If he tells you to fast then fast, and if he tells you not to fast then do not fast.
With regard to making up missed fasts, if your sickness is temporary then you have to wait until Allaah heals you, in sha Allaah, then make up the days that you did not fast, because Allaah says (interpretation of the meaning):
“but if any of you is ill or on a journey, the same number (should be made up) from other days”
But if your sickness is ongoing and there is no hope of recovery, then you have to feed one poor person for each day.
For more information on how to feed the poor, please see the answer to question no. 39234.
We ask Allaah to decree healing and well being for you and for all the sick Muslims.
And Allaah knows best.