
Dang Wangi police chief Noor Dellhan Yahaya said the boy had been taken by his family to Hospital Universiti Kebangsaan Malaysia for treatment.
The boy, who was stabbed in the stomach and on one of his thighs, was in stable condition, he said, adding that police received information about the incident at 5.40pm yesterday.
Subsequently, they detained the man at a unit in the same flats.
Noor Dellhan requested those with information relating to the incident to contact the police.
The case is being probed under Section 326 of the Penal Code for intentionally causing grievous hurt with a weapon.
If convicted, the suspect faces a maximum jail sentence of 20 years, and a fine or whipping.