I am no longer a Christian, however I recall the matter of the 'son judging the father' refers to the son posthumously condemning and chastising his father.

This sentiment is similarly upheld in Islam: that a good son prays to God for mercy for his deceased father if he is a believer but has sins.

As well, the son should not judge his father's past deeds or mistreatment given that all men are subject to sin and weakness. The good son considers the good, mercy, provisions which God provided through the son's father.

However, as a legal guardian of a parent who is mentally disabled and no longer capable of mature discretion, a son is indeed responsible for 'guarding' the father based on the known moral standards, hopefully they being up to God's standards.

What can a son do? Demand strict standards of supervision to meet his standards. Why? Should anonymous sex between parents with diminished mental capacities be permitted out of 'mercy'?

Not if you believe there's a After life.