He is afraid that he may become sick because of being underweight; is he allowed not to fast during Ramadan?
Question

                I am nineteen years old and I weigh 41 kg. It is very easy for me to lose more weight when I fast. Fasting always makes me weak and I start to look pale and debilitated. In addition to that, next week I will start my studies at University, which is about an hour away from where I live. Moreover, the day is very long here in Canada, and I also work during the weekend. My question is: is fasting obligatory in my case? Or is it permissible for me not to fast and to make it up later on, on the days that I think are suitable?.
Praise be to Allah.
      There is a shar‘i concession which allows breaking the fast 
      in Ramadan for one who is sick and unable to fast, or one who fears that 
      he will become sick or is his sickness will get worse. Allah says 
      (interpretation of the meaning): “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” [al-Baqarah 2:185]. 

    Ibn Qudaamah (may Allah have mercy on him) said in 
    al-Mughni (4/403):

     The healthy person who fears that he will become sick by 
    fasting is like a sick person who fears that his sickness will get worse 
    with regard to the concession allowing breaking the fast, because the sick 
    person is only allowed not to fast for fear of what fasting may lead to of 
    the sickness getting worse or lasting longer. So the fear of becoming sick 
    comes under the same category. End quote. 

    Al-Zayla‘i said: The healthy person who fears that he will 
    become sick by fasting comes under the same category as the sick person. End 
    quote. Tabyeen al-Haqaa’iq, 1/333 

    Based on that: 

    If fasting will definitely make you sick or will cause you 
    physical harm because of being severely underweight, then in this case it is 
    allowed for you not to fast, but you have to make it up when you are able 
    to. 

    To decide concerning this matter, it is essential to consult 
    trustworthy doctors. 

    As for merely getting tired or exhausted, or feeling weak 
    because of fasting, that is not an excuse for not fasting, because fasting 
    -- especially on hot and long days -- is not free of some hardship. 

    The scholars have stated that those who do hard (physical) 
    work should start their day fasting, then if they encounter severe thirst or 
    hunger from which it is feared that harm will result, it is permissible for 
    them to break the fast, but they must make up the days on which they broke 
    the fast. It should be noted that study and work are not excuses for not 
    fasting. Fasting is one of the pillars of Islam and one of the greatest 
    obligatory duties of this religion. So it is not permissible for the Muslim 
    to be so heedless about it that he gives precedence over it to study or work 
    that he has no urgent need to do and which he can do without. 

    For more information please see the answer to question no.
    43772 and
    65803. 

    And Allah knows best.