He has a kidney disease and the doctor advised him not to fast

If a trustworthy Muslim doctor has determined that fasting will harm you and has told you not to fast, then what is prescribed is to avail yourself of the concession granted by Allah, may He be glorified and exalted, Who 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”

[al-Baqarah 2:184]. 

Ibn Katheer (may Allah have mercy on him) said: 

i.e., the one who is sick and the one who is travelling do not fast when they are sick or travelling, because of the hardship that will cause for them. Rather they may break the fast, and make up the same number from other days. End quote. 

Tafseer Ibn Katheer, 1/498 

The individual should not put himself through hardship when Allah has granted a concession. The Messenger of Allah (blessings and peace of Allah be upon him) said: “Allah loves for His concession to be used just as He hates to be disobeyed.”

Narrated by Ahmad, 5832; classed as saheeh by al-Albaani in Saheeh al-Jaami‘, 1886 

If the sickness is one for which there is no hope of a cure, then the sick person should break the fast and feed one poor person for each day; if there is the hope of a cure, then he should make up the days he did not fast after he recovers. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 

The scholars divided sickness into two types with regard to fasting: that for which there is hope of recovery, in which case the individual should break the fast and make it up after he recovers; and that for which there is no hope of recovery, in which case the individual should feed one poor person for each day, and this feeding of the poor takes the place of fasting. End quote. 

Fataawa Noor ‘ala al-Darb by Ibn ‘Uthaymeen, 48/216 

The scholars of the Standing Committee for Issuing Fatwas were asked about a woman who had an operation before Ramadan began and Allah did not decree for her to fast before she had that surgery. The operation was done to remove one kidney completely and remove stones from the other kidney, and the doctors advised her not to fast at all for the rest of her life. 

They replied: 

If a trustworthy Muslim doctor has determined that fasting will harm her, then she should break the fast and offer expiation for each day of Ramadan by feeding one poor person half a saa‘ of wheat, rice, dates or similar local staple food. It is not permissible to give the expiation in the form of cash. End quote. 

Fataawa al-Lajnah al-Daa’imah, 10/182-183 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: 

I had surgery on my left kidney at the beginning of last Ramadan and I did not fast that month, because I cannot do without water even for half an hour, and I have not made up those fasts until now. What do I have to do?  

He replied: You do not have to do anything so long as you are unable to fast. If this condition continues and the doctors say that you have to drink water within this short time, then you do not have to fast. As it seems most likely that this will continue to be the case, you have to feed one poor person for each day. End quote. 

Fataawa Noor ‘ala al-Darb by Ibn ‘Uthaymeen, 216/40 

Based on this, what is prescribed in your case is to break the fast and feed one poor person for each day, if the doctor tells you that you will not be able to fast in the future. 

If you will be able to fast in the future, then you should break the fast and wait until Allah heals you, then make up the days that you did not fast. 

For more information please see the answer to question no. 12488 and 23296. 

And Allah knows best.