Keep him for a day. See if classmates or teachers test positive.
I’m not a panicker myself (I’m flying with DCs to Hong Kong tomorrow!) but there is a cluster of cases in HK where traces of the virus were found on a water tap and on a book at a temple, where many worshippers are infected. The virus was present AFTER disinfection had taken place at the temple.
This coronavirus is a tenacious bugger.
You are clearly worried. Your worry is not irrational. Keep your kid home for a day. See what news come out.
More importantly, remind DS of hygiene rules:
-don’t touch your face (this takes self awareness, as we do it all the time - hundreds of times a day);
-bin tissues immediately after using;
-cough in crook of elbow, not in hand;
-wash hands or use sanitiser all the time - every time the thought pops in your head, or at least on the hour.
-stay away from anyone coughing or sneezing. At least 2M away.