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.
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.