I agree with the above commenter—sounds like someone else orchestrated the fake death. I immediately thought of a woman who had just lost her son and took Michael as a replacement. Don't know why, but it lit up like a lightbulb in my head.

I think it's very believable for him to have run away and made it on his own, though. Overnight, he could hop on a train (or a plane if he somehow had a phony passport) and get hundreds or thousands of miles away from home before anyone realized he was gone. Once they realized he was dead they'd stop looking for him; no one would think to ask the train station or airport if they saw a little boy taking a trip the night before. Then he could live on the streets of a big city like New York for awhile—amongst all the other homeless people, he wouldn't be very noticeable.