Ancalagon
Dusty Dragon
Although brain storming a new patron would be fun, did you talk with the player about this?
Absolutely! It would be a jerk move not to.
Interestingly, he's decided he's going to roleplay it out - he doesn't want advanced knowledge about the new powers etc - he'll decide, on the spot, based on who the new patron is: a Topaz Dragon (he doesn't know that either). That's commitment to RP!