A teen discovers a dark secret about his family, he must go on a perilous journey in order to save not only himself, but an entire world.

If you state the dark secret you have also imply why the teen just doesn't call the cops. A little tricky but it will make the story ring true or at least not feel dumb.

as dpg says, way to vague.

Tell us the secret since it is the incident that get's the story going

Let us know specifically what he intends to do.

Hope that helped, good luck with this!

"A dark secret" is non-specific, vague. Heroic characters are always discovering dark secrets. That's part of their job description. It's in the standard Hero checklist of action items. (Right below "Be curious" and above "Save the world")

What makes this dark secret unique, so urgent that the fate of the world hangs in the balance? That only he can save the world?

And what is the relationship between the secret and his character flaw? What personal issue will the mission compel him to confront? How must he first save himself before he can save the world?