Did you ask him to sign a message from one of the Satoshi wallets?
I think the demand should be all of Satoshi's addresses, not just one. Given the scenario above we can make it harder for people to steal satoshi's identity...
My personal opinion of the minimum requirements:
1. Sign bitcoin message using key from address in genesis block. Use proper sentence and grammar with today's date and a quote from any news headline from today. (We can fudge a bit and consider it if it's signed within this year I guess.)
2. Sign bitcoin message using key from address in block 170 (this is the block where Satoshi sent coins to Hal Finney.) Note: Sign the message using the address the transaction was sent from, not the coinbase address because it's possible Satoshi did not mine this particular block, just that his transaction is in it.
3. Sign bitcoin message using as many keys from as many addresses within the first 200 blocks, the more the merrier, but I think 10 should suffice. Satoshi himself would have mined at least 10 of the first 200 blocks.
4. Sign PGP/GPG message.
5. Send us all in this thread a bribe: 1 full BTC each is nice.
Then we can start talking.