Praise be to Allah
We ask Allah, may He be exalted, to grant you a full recovery and well-being.
You should seek the advice of the specialist doctor who supervised your treatment.
If he confirms it you are able to fast, and that it will not have any impact on your health and full recovery, then this will put your and your parents’ minds at rest, by Allah’s leave, and then you will be able to fast, after seeking the help of Allah.
But if the doctor thinks that fasting will be too difficult for you, and that it may cause you some health problems, then it is better for you not to fast, and then make up the fasts after Ramadan, when you are fully recovered, by Allah’s leave.
Ad-Dardeer al-Maaliki (may Allah have mercy on him) said:
It is permissible not to fast in the event of sickness which it is thought – on the basis of the view of a knowledgeable doctor – will be exacerbated by fasting or recovery will be delayed thereby. The same applies if the sick person will suffer hardship and exhaustion if he fasts.
End quote from ash-Sharh al-Kabeer ma‘a Haashiyat ad-Dasooqi (1/545)
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
What makes it permissible to break the fast is that which the sick person fears may exacerbate the sickness or delay his recovery. This is what makes it permissible to break the fast. So if he fears that it may exacerbate the sickness or delay his recovery, or cause undue hardship, in the sense that the sick person will become exhausted if he fasts, all of this makes it permissible for him not to fast.
As for that which is not affected by fasting, such as toothache, pain in the finger and so on, then this does not make it permissible to break the fast, so long as there are no consequences that will result from that. As for simply having a toothache, it will not help a person if he breaks the fast. But if the doctor tells him: If you break the fast and eat something, this will relieve the pain of the toothache, then in that case we say: There is nothing wrong with it, because sometimes lack of food may prolong sickness and pain; and the same may be said concerning eye pain, for example. If fasting will affect it, then he may break the fast, but if it will not affect it, then he should not break the fast.
End quote from Ta‘leeqaat ‘ala al-Kaafi by Ibn Qudaamah (3/123).
The mere fact that a person is below his ideal weight is not a reason for not fasting; rather he should look at what may result from fasting of hardship, exhaustion or harm, and whether he can make up for lack of nutrients due to fasting by eating at night or not. It is essential to consult a trustworthy doctor and find out what advice he can give you.
For more information, please see fatwa no. 140246.
And Allah knows best.