If he knew this, he would have been very sad.

A sentence: If the assignment was not due on Wednesday, I would have been able to come.

This is a counter-factual conditional, which refers to a past thing, if-clause or main-clause. The usual way of teaching this so-called third conditional is IF+PAST PERFECT, I + HAVE+ perfect, but when the meaning is clear, the tense of the if-clause can be replaced by the simple past tense, so the above sentence does not have to change to HAD NOT BEEN DUE. But do you think the sentence below is OK?

if he knew this, he would have been very sad.

This is my reason for thinking it not acceptable, if it is a counter-factual: in that sentence, it is more of a speculation: if he now knew this thing, he must have had the experience of being very sad at that time. So if it is a counter-factual, KNEW must be changed to HAD KNOWN.

What do you think?