- What should he have done?
- What should he had done?
Could you tell me which one is correct? (If any.)
What you want is
Keep in mind that your basic present indicative form is He should [VERB], where the lexical VERB is in the infinitive, uninflected form. For instance:
When you make a question of this, you put your interrogative at the front, keep the modal verb should in second place, and move the Actor, he, after the modal verb:
Now—when you cast your basic form into the past, you employ the present perfect form, using the infinitive of the auxiliary HAVE plus the past participle of the lexical verb:
And the process of turning that into a question is the same as before—put your interrogative at the front, keep the modal verb should in second place, and move the Actor, he, after the modal verb:
You can only say the first. That is because a modal verb such as should is followed by the plain form of the verb such as have, and not by an inflected form such as had.