He had them before the first Gulf War. Saddam was disarmed afterwards and he never had a chance to reacquire them. There were no WMDs in Iraq after the first Gulf War and the UN had an extensive inspection program in place to ensure it continued. Over 700 site inspection reports and not a scrap of evidence of WMDs.
There was no reason to assume that the UN had gotten all of Saddam's WMDs, and like I said before, Georges Sada said the weapons were moved to Syria right before Iraqi Freedom kicked off.