“Cry and howl, son of man, for it (the sword) is against my people, it is against all the princes of Israel. They are delivered over to the sword with my people. Smite therefore on your thigh. For there is a trial. And what if it condemn even the rod? It will be no more, (or alternately ‘what if even the rod that condemns shall be no more?'), says the Lord Yahweh.”
Ezekiel is not to exult in God's judgment. Rather he is to cry aloud and wail like a mourner. For it would be God's erstwhile people in Jerusalem/Judah, and their princes, who would be smitten, for they are undergoing trial. And that period will also result in the rod itself being condemned so that ‘it will be no more'. The puzzle of how Nebuchadnezzar could be used as God's ‘son' is solved, for having condemned others he will then himself be condemned.
‘Smiting on the thigh' was presenting a picture of grief and despair. For this whole idea of God using such an instrument see Habakkuk.