Signed message or this is just another PR stunt for whatever the reason this man may have.
I call BS and all future Satoshi-wannabes who are not able to sign a message and happen talk themselve around doing so.
Edit: Words/grammar. Also: Yes, obviously moving a fraction of the old coins would be another good way of providing proof too. But I think we'll never see this happening.
  
The real Satoshi could do a move to show everyone, that this is just a Satoshi-wannabe. But I don't know. Reacting to such a thing would be a risk too.
If I would be the real Satoshi - I would never ever provide any proof, never move any coins, never draw attention to myself and just let Bitcoin be.
I don't know if it would be technically possible - but if the real Satoshi would sign a message + move coins and tell the public, that they still haven't found the real Satoshi, wouldn't that put the real Satoshi's hidden identity at risk? Couldn't it somehow be tracked, if some 3 letter agencies would have let's say 50 suspected persons they are constantly watching for doing such a move and then just bust him? That's at least what I would do, if my task would be to find the real one - get people's and Satoshi's attention for such a made up thing via the media and just wait which of the suspected real Satoshis reacts in which way. Also if some random guy would let's say want to get this award for himself, the real Satoshi may be in need to act and dox him as a fake Satoshi - because it would not be fair to claim the award as a Satoshi-faker. (If that even might be one of his goals some day)
---> If my crazy theory might be just a little bit correct we will never see any proof for Satoshi's identity ever. None of the old coins on the move, no signed message, nothing. If Satoshi would let himself drive by emotions, he might do it. If he is intelligent enough to not do anything, these kind of stories will just repeat and repeat without any real proof at all.
 
Also apologies for my bad English and grammar mistakes.