If any stockpiles are beyond his control then he ought to have mentioned it already, and he certainly should do so immediately.
It would be quite an oversight though. The rebels haven't taken over any large military installations by surprise, those which have fallen have all taken weeks of fighting to get at. So any sites with such weaponry that they do threaten should easily be evacuated of all high value materiel. If evacuation is not possible, then destruction through adulteration is a simple matter.
The truth is that if the rebels have any noteworthy stocks of nerve gas agents, then the Syrian army is catastrophically useless.