If your nephew didn’t access that paper for years, it doesn’t mean someone hasn’t. Was it stored in a box with glitter nail polish on the lid ? Just something that can prove the location wasn’t opened without his knowledge. Believing it, is not enough.

Could he has noted those words on a block notes and the piece of paper below it recorded an imprint of those words that were recovered be someone else?

Have fun being a Sherlock! But I guess not much can be done.

Not sure the police would consider such a case as a valid theft (at least not yet)