Did this the other night. I was blasting him with arrows making jokes like "Yeah, gonna be everlasting DEATH for yo ass" then he got to a sliver of HP and wouldn't budge. I decided to walk around him, chopped off his tail, then he proceeded to ask me if I wanted to join the covenant.
So yeah, you can still join the dragon covenant if anyone was wondering. --- Youtube Channel - http://www.youtube.com/user/anatomy2k7 - Videos of Recently Released Games
Before NG+ I went around killing all the NPCs and such and got a nice surprise at the tail end of this one. Unfortunately he can't be killed afaik, was a disappointing end to my slaughter. --- PSN: Waggonforce Feel free to add =)