The fire broke out inside an East 57th Street home just after 6 a.m.
Fire trucks navigated their way on a plowed street, around at least one abandoned vehicle, to get to the building.
The 8-year-old boy was trapped in the house and pronounced dead at the scene. Four others were treated for minor injuries.
An off-duty Department of Sanitation worker driving to work was alerted to the blaze by a screaming woman who ran out of the house.
The FDNY said at news conference that the response time was 4 1/2 minutes, which is considered good. They say the snow did not impact the firefighting effort and that the hydrant was available.
The family reportedly did not have a smoke detector, and had double-bolted locks on the door with a key needed to open both sides. The FDNY says that is not a recommended door lock and did slow the family's ability to escape.
The cause of the fire is under investigation.