If it was me I would take him to the ER and have them give him an IV to help with the vomiting. You can also see if they could find anything at the hospital that the doctor is not really looking for or at.
Water will not replenish what your son is losing thru the vomiting and watery BM's. You need to give him orange juice and/or gatorade which wil replenish his electrolytes. But if he isn't keeping anything down and has the bad BM's this will not benefit him! Every virus is different, some are 24 hrs, some 48hrs, others a week, it all depends on what he has. He has to be getting dehydrated by now, so I would insist that they give him something for this. There are many medication to stop vomiting, I assume they aren't giving him anything?