Ali Saqr, 20, had reported his mother, Lina, to ISIL authorities in Raqqa "because she tried to persuade him to leave ISIL and flee the city," the Syrian Observatory for Human Rights said.
Authorities subsequently arrested the woman and accused her of apostasy, the monitoring group said.
On Wednesday, she was shot to death by her son "in front of hundreds of people close to the mail service building in Raqqa city," the Observatory added.
Observatory head Rami Abdel Rahman said the woman, who was in her forties, was living in the nearby town of Tabaqa but worked in Raqqa city.
The incident was widely condemned online by social media users, AFP reported.
Raqqa is the de facto Syrian capital of ISIL's so-called "caliphate," the territories it controls in Syria and Iraq.
In ISIL-held territories, using child soldiers and "owning slaves", which are typically sexually abused, are both legal.
The ISIL Takfiri group has been committing heinous crimes against all ethnic and religious groups, including Shiites, Sunnis, Kurds, Christians, Izadi Kurds and others in Iraq, Syria and several other countries.