A.P.
There are hundreds (if not thousands) of gastro-intestinal virus' that could be causing this. Testing is the ONLY way to diagnose.
So, since it's likely to pass in a matter of days to even a week or more, the only thing to do is treat the symptom.
Unless you want expensive, unnecessary tests which will tell you "your son has ABC Virus, give him bland food and water the next few days and it will pass".
As opposed to your current answer "your son as a virus, give him bland food and water the next few days and it will pass."