You should post a signed message to prove that the new account is really you and not an impostor.
Not really needed in my opinion. Many people create new accounts and it would get very annoying if everyone signed a message in meta or gave us a notification that they were doing so
Yes, not necessary.
But if he already opened a post to write about his change of name, it seems like he wants people to know about it. (I don't get why)
And if that is the case, a signed message acts as proof.
I don't see what reason is there for publicly showing that you're making a new account, but if you really want to, a signed message will make others more sure of the change.