Is the reward for remembrance (dhikr) counted for one who prayed Fajr and sat remembering God until sunrise, then prayed two rak'ahs, but was interrupted by his sick father to serve him?

You must answer your father's call because serving him is obligatory, and prioritizing an obligation over a supererogatory act is better. The reward for serving a sick parent is greater than sitting in the prayer room until sunrise. A Muslim is credited with the reward of the good deed he used to perform if an impediment, such as serving his father, prevented him from doing it. This is based on the Prophet's (peace and blessings be upon him) saying: "If a servant falls ill or travels, the like of what he used to do when he was a resident and healthy will be written for him."