He wants to delay making up Ramadan fasts until winter because the days are shorter
Question

What is the ruling on making up missed Ramadan fasts in winter when, as we know, the days are short?.
Praise be to Allah.The one who does not fast some days in Ramadan is obliged to make them up before the next Ramadan comes, whether that is in the winter or otherwise, because Allaah says (interpretation of the meaning):
“but if any of you is ill or on a journey, the same number (should be made up) from other days”
[al-Baqarah 2:184]. 
And it is proven that ‘Aa’ishah (may Allah be pleased with her) had days to make up, but she did not make them up until Sha‘baan came because of the status of the Prophet (blessings and peace of Allah be upon him). 
And Allah is the source of strength. May Allah send blessings and peace upon our Prophet Muhammad and his family and Companions. End quote.