Ok, I just did this earlier today. I ran through the whole cave and saw nothing. Turned round and went back through. Standing near the end chatting with another player if they knew where the boss was...and bam! there he was.
He spawns in the circle of blue-ish candles at the end of the dungeon. The end of his tunnel has a cot in it.
I think his respawn timer may simply be longer than some of the other public dungeon bosses.