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.