Should he have been accused?
Should he had been accused?
Which one is correct?
The only valid one is:
The reason is that we are asking the question 'should' in the present.
All perfect tenses comprise a trigger action/time and a finish action/time.
Present Perfect comprises all the time between the trigger action ('have been accused') and the second and finishing action/time ('should?').
So Past Perfect is invalid, as that would entail asking the question in the past.