Is it permissible for him to prepare food for a non-Muslim son during the day in Ramadaan

Praise be to Allah.

It is not permissible for a Muslim to offer food during the day in Ramadaan unless that is to a person who is excused for not fasting, such as one who is sick or travelling. But if he has no excuse then it is not permissible to give him food, whether he is a Muslim or a kaafir. 

As for giving food to a Muslim who does not have an excuse, that is clear, because he is sinning by not fasting, and it is not permissible to help him in his sin. 

As for the non-Muslim, he is also sinning by not fasting in Ramadaan, because the minor issues of sharee’ah are addressed to the kuffaar as is indicated by the evidence, and this is the view of the majority of scholars. What is meant by the minor issues of sharee’ah being addressed to the kuffaar is that the kuffaar are obliged to pray and fast and do all duties, just as they are obliged to observe the basic principle, which is to believe. So the kaafir will be called to account on both counts, the basic principle and the minor issues, and he will be punished for failing to observe the minor issues just as he will be punished for failing to observe the basic principle, even though observing the minor issue is not valid unless the basic principle is first observed. 

So the kaafir who is able to fast must fast, just as he must become Muslim. If he does not fast then he is sinning by breaking the fast. Hence it is not permissible to help him in this sin, rather he should be left to prepare his food for himself. 

Al-Nawawi (may Allaah have mercy on him) said: The correct view which is followed by the scholars and the majority is that the minor issues of sharee’ah are addressed to the kuffaar, and silk is forbidden for them just as it is forbidden to the Muslims. End quote from Sharh Muslim, 14/39. 

Al-Ramli (may Allaah have mercy on him) narrated in Nihaayat al-Muhtaaj (5/274) that the scholars ruled that it is haraam to sell food to kaafirs during the day in Ramadaan. 

See also question no. 49694. 

We ask Allaah to guide these two young men and to set their affairs straight. 

And Allaah knows best.