The baby, Jayden Smith, was observed vomiting and having difficulty breathing when police responded to the Bragg Street apartment just after 4:30 a.m. last Thursday.
He was rushed to Coney Island Hospital and then transferred to Maimonides Medical Center, where he was pronounced dead later in the day.
Detectives were notified Tuesday that the death has been ruled a homicide, the cause of death being blunt force trauma to the abdomen. Mom Geraldine Smith was questioned and charged with assault, endangering the welfare of a child and causing an injury to a person under 7 years of age.
The boy's twin sister was removed from the home and is being looked after by relatives.
The investigation is continuing, and additional charges are possible.
The city's Administration for Children's Services investigated four prior incidents in the apartment in 2011.