A 2-year-old Bronx boy was still alive when he was “callously” thrown into a river by his father — who later boasted to the child’s worried mother, “Shut the f—k up! I threw that n—a into the river” when she confronted him about where the boy was, prosecutors said at the dad’s arraignment.Video reviewed by investigators allegedly showed Arius Williams launching little Montrell Williams into the air and throwing him into the river just before midnight May 10, the day he went missing, ADA Astrid Borgstedt said in Bronx Criminal Court on Thursday.Moments before the tot was tossed off the Bruckner Bridge, he appeared to be alive and standing upright in just a diaper, prosecutors said.Arius Williams allegedly told the mother of little Montrell Williams, “Shut the f—k up! I threw that n—a into the river,” as he threatened her with a knife on Sunday, prosecutors revealed in court.Kevin C Downs forThe New York Post“The video surveillance shows clearly Montrell Williams was in the defendant’s arms,” Borgstedt said — noting footage from after the incident showed Williams walking by himself.