Assad has already lost much of Syria, making his concession of that a mere formality. And if we really want to stop the slaughter, this might be the way to go:
[The] US should back the partition of Syria into a Sunni majority state and an Alawite rump state in the northwestern quadrant of the country, where the Russian navy station happens to be located. The Kurds should get autonomy, just like their Iraqi compatriots.
He adds getting our Sunni Arab friends to purge the rebels of jihadis and attacking Iran's nuclear facilities and other key pillars of the regime.
I've mentioned it myself:
We'd need to give the Alawites and their Christian allies a province they demographically dominate on the coast and mountains of western Syria; the Kurds would get a province in the northeast; and the Sunni Arabs would get the rest.
Damascus would be a federal enclave.
Power would be devolved to the provinces, including foreign affairs excepting some symbolic aspects reserved to the mostly nominal federal government. Assad could become president of the Alawite province and carry on as he has.
The Kurds and Sunni Arabs would get their own provinces. Perhaps the Sunni Arabs get multiple provinces based on existing provinces or groupings of them. The various rebel factions could be bought off with control of these provinces.
Multiple Sunni Arab semi-autonomous states would allow a pro-Jordan entity near Israel and Jordan.
And perhaps add a provision for a referendum on keeping Syria whole or allowing provinces to declare independence after a decade and after talks to divide up assets and debts.
Talks two years ago would have kept Assad in control of Syria. Now talks would ratify Assad's loss of most of Syria.
I'd add working to remove Assad from power in his rump regime, so that a Rump Alawite realm is just a stepping stone to total defeat for Assad.