Maybe a brief video showing going to it, browsing through his posts to show other things he's said that give credence that it's his, and opening the post shown in the screenshot? An explanation of how the OP got past JA's privacy settings would be helpful. Neither "prove" it's him, but are better than a lone jpg.

How did we know it was JA's real Insta the first time around? If we can't do the same thing now, we have to leave this in the realm of unconfirmed and possibly disinfo.