He spawns by the entrance to the chamber where you find the second half of the rod... so you'll only see him when you head towards the exit.
However, if you save your game after you assemble the rod but before he arrives, BEWARE! If you reload that save, the spawn trigger gets broken and he'll never appear, forcing you to reload an earlier save. You should always save before opening the container and not overwrite that save until after he's dead.
If you think that's what happened to you, then your best bet is to reload an earlier save.
Yep. I am pretty sure that is what happened to me AND I think i ended up overwriting my earlier save. I tried to use the old debug codes to see if I could force a respawn, but they do not seem to be working.