I would think if that kid was Blake he would have mentioned that Batman had saved his life and that he overheard Rachel calling Batman "Bruce" on the rooftop in the Narrows in Begins
and that that was how he put two and two together about Batman's identity.
When I was tossing out theories about how Blake made his wild ass guess earlier in the thread, I postulated that that kid was later orphaned and he was living in the same boy's home as Blake, and that he shared his story about meeting Batman years earlier and overhearing his name. Blake then later made the connection between Bruce and Batman. It's more plausible than "I saw your anger about your parent's death and boom, I knew you were Batman."