It seems likely that he really is alive, but AFAIK, there's been no recent signature from a wikileaks private key. People on reddit seem to be convinced he's not in control of the keys anymore, which is somewhat disturbing.
Really if you don't believe he's alive for the reasons I posted above, it's likely that nothing will convince you!
Our concern is that that would reduce the proof of life to a specific methodology. Key management is complex. Primary proof is social proof, lawyers, Pilger, et al. It would be extremely costly for adversary to create conspiracy involving all these actors.