If he can really move some coins in known satoshi addresses, that could be a proof.
Signing a message would be the only way to verify ownership, sending coins also is good but who's to say its actually him, and you can't include a message in the coins you send unless its on blockchain, but thats a centralized way of adding notes, only blockchain uses that.
Signing a message timestamped, and with his username or real name would be the only way that we could really verify.