provide proof of the queen giving him the land, mansion, and title. There has to be paper on that, and it will probably stand up as a legal name change. Prove that he is thirteen and has no medical degree or license. Provide testimony from others that Lady Elizabeth has acknowledged him as Ciel for three years. If it comes to down to a trial, he can tell his story in court about seeing his brother killed, so he would have no motive to revive him because he watched that temple burn down.
He could appeal to the emotions by explaining that he knew no one would be happy if he returned, but that if Ciel did, they'd be overjoyed.
It occurs to me that ourCiel said he was fine with being eaten by Sebastian because he didn't believe he could sustain the role of being Ciel for much past three years. He might be just as happy to let Ciel#1 take the reins
(but I doubt it)
How is Ciel going to prove that he's not the one who was reanimating corpses?