I have another thought.
What if Assad did have him.
But “traded” him for someone.
Well anything is possible but the most likely explanation of things is usually the simplest. So I think the ISIS elements had him all along.
Assad may be a dictator, but his dictatorship did not try to commit mass murder on different religious elements within Syria. The alternative dictatorship/caliphate that ISIS would set up in Syria on the other hand would add a newly expanded dimension to the definition of MONSTROSITY.