Scammers are getting very innovative now, and they are very brave to post it here I'm getting similar messages on Telegram and just ignored it, but asking him to sign his Bitcoin address that has 50 Bitcoin on it, is a good proof that his offer is legit but won't guaranty that he will not scam people.
You right, the next step is asking if can escrow founds but 1st he should sign a message from an address with almost 500k $, let's see what happens but I'm pretty sure what will be the result.