Take the dog back to the vet. Medicine is not a perfect art - meaning that due to how many diseases are out there that share similar symptoms, the treatment is usually to treat the most common cause (illness) and see if that works. If not then the more expensive treatments for other possible issues, including tests and labs are done. I will assume that your vet wants to save you the 6, 7, 8 hundred dollars for tests and treated the dog for a common ailment of the species/breed first. If nothing happens in the time alloted by the vet (who should have told you what to expect in the way of treatment and the dog getting better and how long) then you need to see the vet again since a short list of potential ailments are now ruled out and this opens the door to another list of ailments. Vets are harder pressed to perform perfect diagnosis more than human doctors since they can not talk to the animal and have the animals input as to 'where it hurts' and what it 'feels like'. Even in human medicine such communication does not help much - Doctors attempt to ask lots of questions to rule out certain ailments - but even then wrong diagnosis is possible. Mind no illness affects two patients exactly the same - thus it is always possible that one patient may seem to be suffering from one illness when they are suffering from another.

try starving for 24hrs with only fresh water avalable. then try him slowly on just small amounts of cooked rice and chicken then slowly move on to dog food. if doesnt improve in couple of days defo take him back to vets. also has he been wormed because it can effect them like that aswell. maybe he could of picked something up from the kennels. it could also be the dog food you have him on might not agree with him

whatever prokoin is...it sounds like it would give a dog the runs.