Praise be to Allah.
If there is a necessity which means that this child cannot be circumcised at the time prescribed by sharee’ah, it is permissible to delay it, because Allaah says (interpretation of the meaning):
“And do not kill yourselves (nor kill one another). Surely, Allaah is Most Merciful to you” [al-Nisaa’ 4:29]
“Allaah burdens not a person beyond his scope” [al-Baqarah 2:286]
And the Prophet (peace and blessings of Allaah be upon him) said: “If I command you to do a thing, do as much of it as you can.”(Narrated by al-Bukhaari, 6858; Muslim, 1337). And there is the shar’i principle that cases of necessity make things which are ordinarily forbidden permissible.
Circumcision does not usually result in harm, but if the doctor who has told you not to have your son circumcised knows that it could cause real harm, there is nothing wrong with waiting until the child is able to undergo this procedure.
“One of the things which cancels the obligation of circumcision is when the child is weak and is unable to bear it, so much so that that his life would be in danger. If his weak state persists, he will be excused for not being circumcised, because at most, circumcision is (obligatory) but because he is incapable of doing it, he will be excused. This is the same as in the case of all other obligatory duties.”
“In Sharh al-Hidaayah it says: … he should be prevented from doing it, and there are many other similar examples, such as doing ghusl with cold water in very cold weather when one is sick, or a sick person fasting when his life may be placed in jeopardy if he fasts, or carrying out the hadd punishments on someone who is sick or pregnant, etc. All of these are reasons why the action should not be carried out, and they make it no longer waajib (obligatory).”
And Allaah knows best.