In case none of those work...
Leave the Big 4 and work very hard to be successful client side, then ask that this person be staffed on a project. Sure enough you should learn their name in due course.
Pickpocket them in the corridor and get name from driving licence.
Ditch the hair dye, grow a few greys, then tell him you are in fact his father. Tell him you wanted to name him Mercedes or Crystal or something cos you thought he was a girl and ask him what name her prefers to use (a bug*er if he actually wants to be called Mercedes or Crystal etc obviously)
Take him out drinking, drug him, then make sure he wakes up naked in your bed. Tell him to disclose his name forthwith of that video of last night will find its way to his wife. Suggest sleeping on the sofa yourself. Sacrifices called for.
Tell him you have an anagram fetish and ask him to make an anagram from his name. If he answers to Bob you just gotta accept this is not going to work.