。
To tell the truth, he's not human.
Literal
True [possessive-の] thing [object-を] say-[conditional-ば] he [topic-は] human not.
本当のことを言えば — the ば-conditional 'truth-telling' opener, followed by a dramatic revelation. 人間ではない ('is not human') is blunt and striking. The sentence could be from a sci-fi or fantasy context — or a cutting insult.