I regret to say he’s gone for good.
Understandable but unnatural robotic word-for-word translation. We don’t usually use 永久に when describing a human being. It even sounds a bit rude except that’s what you intentionally treat the person like a disposable object. The English sentence suggests that his departure is a loss for the speaker.
My alternative translation is:
残念ながら彼が戻ることはありません。(pron. もどる) = (lit.) Unfortunately, he will never come back here.
彼は永久にいなくなった may be used only in the context of ban/ouster/exile.