Are the air ops against IS being done with explicit Syrian permission (all nations not Russia) or because Assad can't stop anyone flying over his airspace? I think there is some kind of back channel deal, but I could be very wrong.
I think Assad has welcomed western strikes although he knew those were mainly for posturing and didn't have much of effect on ISIS(considering very "moot" RoE). So even if he had the means to close the airspace he wouldn't do it IMO, ISIS was and still is the biggest threat to him as all other factions can't rally more than a thousand or two of fighters each at any given time. Those "moderate" warlords seem not to really like each other.