Fellow dads... I need the collective wisdom of the community to pull off a prank 8 years in the making.

When my son was 4 years old he was scared of his room one night and asked me what would happen if a werewolf were to show up. Me, being a dad, told him that he had no need to worry because I knew the secret to kill werewolves and that it was handed down in our family for generations. He asked to be let in on the secret, of course, so I told him that I could not tell him until he was 12 years old. THis has been a running joke in our family ever since. Whenever anyone mentions 12 years old, I get a really serious tone and a solemn look on my face and say "You'll know the secret by then" or if he asks to stay up all night I say "When you're 12 you can finally stay up with me, but I cant let you until then, family rules."

He is 10 now and he knows its a joke, he laughs and tells me "There is not a secret! Werewolves don't exist!" but I persist. and now I have 2 years... I want make it a big deal. I am a leather worker, have access to a maker space, can woodwork etc. I plan on buying (or making) some handmade paper and making a leather bound book that will be a "journal" of the secret. But I need more ideas... should it all be "in character" or should it really be a heartfelt telling of what its been like to be his dad? Both? What all should I include in it? snippets of "werewolf hair" etc? I plan on aging it an addign fake dust etc to it to really sell it... but what else should Ido?