There’s a few reasons I don’t like either “little rumple lost boy” or Peter Pan is his brother theories, but together it’s just too much I think. That’s not a reason they wouldn’t do it though, just a reason I don’t like the idea.
Then again, if they’re casting his father, then they have to find someone to play little Rumple anyway.
Now maybe they want to show little Rumple just to establish that he really understands where PP comes from, that sense of being abandoned so young that you barely remember the people you thought were supposed to love you.
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass