“And when this happens, behold it comes, then will they know that a prophet has been among them.”
‘This' is the further desolation of the land after the fall of Jerusalem as described above. That will further evidence to them that a true prophet of Yahweh has been among them. But the question will continue to be, will the exiles now genuinely respond? As watchman to the house of Israel it was his responsibility to go on blowing the horn.
Others see ‘this' as referring to the coming restoration.
Remarkably this is the last mention of Jerusalem in Ezekiel (apart from a secondary mention in a comparison - Ezekiel 36:38). Jerusalem, having been destroyed, is no longer of interest to Ezekiel. His emphasis is on the restoration of Israel as a whole. He is concerned with the people of God, not with the restoration of Jerusalem.