I can't say I remember perfectly well, it's been awhile.... I don't think he can be knocked down, but there's nothing anywhere that says he can't be, so I'm not sure.
I believe nearly every foe in the game can be knocked down with [Earthbind]
It magically makes foes typically immune to knockdown able to be knocked down, and of course for three seconds. The only exception I can think of off-hand is Abbadon.
