I have not seen him since the very first day after the patch he was added in. Any confirmation on this?
What is the respawn timer on him - that might be a good place to start. I know he has the MotM buff and I assume it's an enhanced respawn rate - like all other raid targets have received.
I've seen him up more recently, but if my experiences are any indication the timer seems extremely long, especially for a mob with MotM.