Is it better for a son to pray in the first row or to pray with his sick father in the back rows on Friday, and is the father's obligation to pray and fast waived?

Your father's accountability for religious duties depends on the extent of his memory and cognitive loss. If he has completely lost his cognitive faculties, he is not accountable and is not required to pray. However, verifying this requires direct interaction with him.

If your father is accountable: Your delay in reaching the first row due to serving him and bringing him to the mosque increases your reward and your dutifulness towards him. If he is not accountable (if he is described as demented or insane): You are not religiously obliged to take him to the mosque; rather, strive to distract him from going. But if he insists on going and does not disturb the worshippers, then obeying him and pleasing him is an act of filial piety. It is not a condition that this be better than the first row, and obedience to him is not obligatory if his command results from a lack of intellect and foolishness.