If a Muslim religious and expert doctor prescribes that fasting is harmful, he can leave the fasting. But, once he is able to fast (in shorter days of winter), he will have to make qadha of the same. But if he is suffering from a disease which does not allow him to fast in any season and he is in a stage of life that recovery is unexpected, he can give fidyah (expiation) of the same. But, he was able to fast in future, he will be asked to keep those fast again. Therefore, if he expects to keep fasts, he should make a will of fidyah in case he could not fast and died and inform the family members of the same.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband