If he did not fake his death, that would seem to mean one of two things: Either "someone else did it for him," or "he actually died." Is there any other choice?
For someone else to have done it for him, then at some point, by continuing to be alive while knowing the world thinks he is dead, then he has to become a willing participant by not saying anything, therefore essentially being responsible for the fake death.
UNLESS... he sits somewhere in a coma or vegetative state ever since then, or he has amnesia and truly believes he is someone else, or is a prisoner of someone. All seem to be pretty wild estimations, but pretty much the only scenarios where his faked death, outside of his control, would continue to be outside of his responsibility.
That leaves us with the other thing. And there wouldn't be much mystery to the point of "how he did it."
