Sorry to say but even if this person learn how to sign a message is the BTC coming back? Are you guys going to get his BTC back for him? Why are you forcing him to sign a message? So that you can believe that he is indeed the rightful owner? What is next after? Let us send his BTC back to him since he is the rightful owner?
OP, I am glad you learned your lesson, you don't have to keep longing this thread, you shared your pain and people don't still believe, leave the post and move on, new readers will learn something from it or also join the group of people that don't believe.
My advice for you now is to get a hardware wallet, it's not even up to $300 to get a solid one, Trezor, Onekey, Keystone, Tangem, D'cent, and so on, these wallets are badass, good for keeping large amount of crypto, now they are doing discount, black Friday's everywhere, act now to avoid future losses, take heart and be strong.
Because it's quite often for newer members to come up with such stories to attract attention and merit as well, in a desperate attempt to rank up. I don't know what to believe. I've read a whole lot of stories here, and a large number of them were fiction. I can't accept that someone who supposedly had 11 BTC, worth approximately half a million dollars, was that of a novice who did not have a hardware wallet, which barely costs $70, and kept his wallet on a shared Windows computer, not even a separate computer or OS for this purpose. I'm sorry, but it doesn't sound plausible.