His passive doesn't give a stack. I autod a target dummy, Qd, and the return Q passive proc. did absolutely nothing for stacks.
I'm also not sure if it's intended that the return Q offers no stacks either. Karthus E offers multiple stacks.
Blue Smite gives no stack either even though it counts as a spell for Electrocute. This is odd. The Runic Echoes proc. also does not despite applying spell effects.