The boy named Bablu Kumar Paswan drowned in the pit near Worli Sea Link at 1.20 pm Friday, said an official of the BMC.
The ambitious 8-lane Coastal Road will link Marine Lines in south Mumbai with suburban Kandivali in the north.
"Locals rescued the boy and rushed him to the nearby Nair Hospital where he was declared brought dead," the official said.
It was being probed how he fell into the pit, he added.
This comes days after the incident the city where a 2-year-old boy fell into an open stormwater drain in suburban Goregaon.
The search by fire brigade and National Disaster Response Force was called off after 48 hours Friday as the boy remained untraceable.
Ever since the death of Dr Deepak Amarapurkar two years ago, the BMC has come in for flak frequently and also faced public interest litigations over open drains, pits and manholes that can become deathtraps during the monsoon.
Opposition Maharashtra Navnirman Sena said the ruling Shiv Sena should take responsibility for this situation.
"When the Shiv Sena takes credit for every new construction (by the BMC) and achievement, why it shies away from taking credit for creating such a mess in the city?" MNS spokesperson Sandeep Deshpande said.