Does he have to refrain from eating on the day he becomes Muslim?
Question

                If a kaafir becomes a Muslim during the day in Ramadaan, does he have to refrain from eating during the day on which he becomes Muslim?.
Praise be to Allah.
    Yes, he has to refrain from eating for the rest of the day on 
    which he becomes Muslim, because he is now one of the people who are under 
    this obligation. This is different from the case when an impediment to 
    fasting ceases to apply; when an impediment to fasting ceases to apply, the 
    person does not have to refrain from eating for the rest of that day, for 
    example, if a woman’s period ends during the day and she becomes pure, she 
    does not have to refrain from eating for the rest of the day. Similarly if a 
    sick person who didn’t fast because of sickness recovers during the day, he 
    does not have to refrain from eating, because it was permissible for him not 
    to fast on this day, even though he is one of those who come under this 
    obligation – i.e., a Muslim. This is different from the case of one who 
    becomes Muslim during the day; he has to refrain from eating but he does not 
    have to make up that day. But those people – menstruating women, people who 
    are sick and so on, do not have to refrain from eating but they do have to 
    make up that day. End quote. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him).