“How do you know if a stomach bug is dangerous? ”

It sounds like an infectious gastroenteritis. These infections are often self-limited but the dangerous ones don't get better quickly and can cause dehydration or other complications. Dehydration, blood in the stool, or other symptoms that last more than a day or 2 should prompt evaluation in a doctor's office. Go to the ER for severe symptoms.

Consult healthcare provider if accompanied by fever and symptoms Persist beyond 24 hrs

The main issue with these symptoms is dehydration so please make sure that he drinks electrolyte solution and if he is unable to do so then please take him to the ER.

High fever, vomiting, and dehydration.

Most of the time a so-called stomach bug will make a person extremely miserable for between 24-48 hours. There can be frequent diarrhea and/or vomiting. The biggest danger is becoming dehydrated. I would encourage your son to take small sips of water or Gatorade (or a similar product) fairly frequently. This usually works better than trying to get a lot of fluids in at one time, which is more likely to cause more nausea and vomiting. Sometimes intravenous hydration becomes necessary. If your son’s eyes are sunken or he hasn’t peed in 6-8 hours or more, he may need fluids. A “stomach bug” is not the only thing that can cause vomiting. Take him to medical care if he has high fever, if he is having hallucinations, if he vomits blood or poops blood, or if he becomes dehydrated.
I hope this helps.

Randall Fisher, M.D.