Subject and predicate exchange？

Why can we say "he is a student" , but not "a student is him".

Also, "he is the best student" and "the best student is him". Both sounds good?

In He is a student you are ascribing the property a student to a definite referent he.

This order cannot be reversed for any construction of this sort, e.g.

He became a good student.

*A good student became he/him.

He made a good student.

*A good student made he/him.

In the second example, both the best student and him have definite referents. This is a specifying use of be, and the order may generally be reversed with the specifying use of be. (CaGEL p54).

He was the man who stole my purse.

The man who stole my purse was him.

This may look a bit odd, but if we add some qualifying information, it looks quite natural, e.g.

The man who stole my purse was actually him all along.