Hello~I hope that by now, he is doing well. It could possibly have been a mild case of the flu, even though the rest of the family did not have it, that doesn't mean he couldn't have caught it.
Sounds like Gastroenteritis, which is usually self-limiting.
The BRAT diet (bananas, rice, applesauce, toast and tea)
might help with symptoms, but only for a short time, as it only offers limited nutritional value.
Best wishes,
Niko