Course it had to be him. It was someone that his mom knew back then, and of all the character we got introduced he was the only one in the age range to make any sense.
The surprise wasn't that it was him, it was that you think the entire time this mother fucker was one step behind from the kids, while in fact he figured that shit out and laid his own trap, and unlike our hero, he has no future knowledge.
I hope to god they don't cut the backstory, or don't cut too much of the feels stuff in the next few episodes, but if the show ends at ep 12, this is going to be one rushed ending.