Response: The ruling revolves around his state. So at the times he is awake and sane he must fast, and at the times he is unconscious or senile he is not required to fast. Let us assume he is senile one day and sane the next, then on the day he is senile he is not required to fast, whilst on the day he is sane he is required to fast.
Shaykh Ibn ‘Uthaymeen
Fataawa Ramadhaan – Volume 1, Page 239, Fatwa No.186;
Fiqh al-‘Ibaadaat – Page 187. fatwaonline.com