This theory has probably be said before so I’m sorry of it has:
I’m going with Bae also. With the help of the Blue Fairy, he was sent to the “The real world,” Like Emma and August, who are capable of entering and leaving storybrook with no harm done to them. August could of eventually ran into Bae, who tells him of Emma’s whereabouts, soon falling in love with her, they have a kid Henry. After the curse is broken August is a normal boy again and contacts Bae.
I’m guessing this is the most obvious. I’m just curious of how Regina got her hands on him.
My other theory is also Peter Pan, who can travel in both worlds. He was probably the one who helped create the spell, along with Tinkerbell. They’re bringing in Captain Hook. It would make sense.