There seems to be a hotfix, the last 3 kills I was there for, general chat was flooded with people saying they have just gotten the mount. It's either been raised or is now a 100% drop chance.
Seems to be a daily boss as I got with him on two different characters the achievments for daily quests with him.The mounts seems more like a 10% chance as I got it on 13th try, the scythe on the 12th and the toy on the 3rd.
Update: Mount and Toy are now 100% off the Boss