Is prayer obligatory for a cancer patient who is taking medication that causes him to lose focus and awareness, and what are the responsibilities of his children towards him?

If your father's lack of concentration and excessive drowsiness are due to medication, and this condition lasts for the entire prayer time, preventing him from performing the prayer with concentration, then the prayer is neither obligatory for him nor does he have to make it up. However, if there are periods of lucidity during which he can perform the prayer with its prescribed pillars and proper form, then the prayer is obligatory for him. Similarly, if he regains consciousness before the prayer time expires, with enough time to perform one rak'ah after purification, then it is also obligatory for him.