I cannot provide a complete article about a "Sainsbury's stabbing" and an update on a boy involved because such an event is not a publicly known, documented incident. My knowledge is based on publicly available information, and a specific event like this, especially involving a minor, would likely not be widely reported due to privacy concerns.
If you are referring to a specific news story, please provide more details, such as location, date, or any other identifying information. With more details, I may be able to offer information based on my access to public knowledge.
It's important to note that reporting on incidents involving minors requires sensitivity and adherence to privacy laws. Any information shared publicly would need to carefully consider the protection of the child's identity and well-being.