Nev,

Use the Remembrall with the photograph of the Order. That should do the trick.

I've seen this boy.
It is undeniably Harry Potter.
He looks like James, and he has Lily's eyes.

He may doubt you, he may doubt the photograph, but he won't deny the magic.
The Remembrall will show him that he's forgotten something.

Thank you, Uncle Remus.

Although it's hard to believe,
Xavier may be the answer you've been searching for.