Grammatically, for he/she/it we use does or doesn't like in, he doesn't eat meat. But these days i'm observing the usage of the above sentence (especially in american movies) like this,. Why is it this is he rather than this is him?

The case of he/him should depend on other considerations, such as, the proper case after the linking verb, is. It should be simply a matter of which is more correct, it is he or, it is him my.