Board Thread:General Discussion/@comment-31845162-20170426215837/@comment-26967699-20170426220700

He probably found out the storybook is actually real because he is the Truest Believer. As we can see in 3.10 flashbacks, Henry realizes Mary Margaret is actually Snow White the minute he gets the book.

If I recall correctly, he also found out his biological mother with the help of a website: "Who's Your Momma?".