It's a little buggy sometimes, but I can't seem to figure out what causes it. I've done him about 4 times, 2 of the times it worked flawlessly, one time adds came at 80 (2 of them) but then none came until 20%. At 20% ALL the adds came that were supposed to come at 40 and 60%. I was fighting him by the port for the 3rd and 4th kills, which is the only thing that I can think of that changed. Maybe it's an aggro range issue?
The 4th time I did him no adds came period until 20%. At 20% all of them came at once!