All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If it was proven by experience, or if the doctor told him that fasting harms him because of such illness, then it is permissible for him to break the fast; Allah Almighty Says (what means): {So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up]...} [Quran 2:184]
However, if this disease is chronic, and there is no hope to recover from it, then he can feed one needy person for each day he breaks the fast. Nonetheless, if it is an illness from which he will recover, then it is obligatory on him to make up for the days he breaks whenever he becomes able to do so. He is not allowed to resort to feeding if he is able to make up.
Allah Knows best.