The solution is to destroy the weapons. The Syrian opposition groups should recognise the value that the international community taking an interest in a catalogue of atrocities committed in what appears to be a very destructive and beyond horrific experience for the entire civilian population. Assad seems unable to release his iron grip on power but ultimately step by step the Russians can be entirely influential it seems. The instant agreement given by the Assad regime means it fears military intervention as it should from a potentially formidable foe (for example, NATO) if it does not comply with the humanist objective of removing the offending weapons from the equation. They could fall into the hands of the opposition groups and it is a strategic disadvantage in the endgame of this war to have them.
Consider the unlikely scenario that Assad is correct in saying that the opposition groups launched the multiple coordinated chemical weapons attacks that genocidal day a couple of weeks ago. Instant compliance is explained in that scenario. Now consider that even if Assad was in control of the chemical weapon attack the danger of such weapons, scattered around the country poses an existential threat to both sides. Removal of them return him to certain dominance.