Carl Ducasse’s family in the Bronx had been planning something big to mark two approaching milestones: his graduation from high school this month, and his 18th birthday in August.

“It was supposed to be a big celebration,” his cousin, Narier Ducasse, 29, said on Saturday. “And now we’re planning a funeral.”

Carl Ducasse and a friend had gone out on Friday to get a milkshake from a bodega around the corner from where he lived in the Mount Hope section of the Bronx, Narier Ducasse said, but his cousin never made it back.

The police said he had been fatally stabbed in the chest during a robbery around 11:20 p.m. near the corner of East 175th Street and Walton Avenue, a block from the apartment where his mother had been waiting for him to return.