If you look at the examples from DWDS, you see that the finite forms of the verb "sein" are almost always followed by something. "Er ist" without a complement is not completely ungrammatical, for instance it might occur in a religious or philosophical text where the author differentiates between "er ist" and "er existiert". It may also contrast with "er war", say "War dein Vater krank? – Er war nicht, er ist [krank]." In everyday speech, however, it sounds usually strange.
On the other hand, "ist er" is fine, because it can be interpreted as an ellipsis: "[Das] ist er."