Police quoting an eyewitness said the mother and son were standing by the roadside and talking when he suddenly whipped out a sharp instrument and stabbed her.
The youth, identified as Abijith, was nabbed by police when he tried to flee the area after the incident.
The woman was admitted to the Medical College Hospital, where her condition was stated to be stable.
Hospital sources said the stab wounds in the woman's stomach and neck were not deep.