Has your son's dog ever been tested for parasites with a fecal test? The first thing coming to my mind is Coccidia.
http://www.animalhealthcare.com/hand...occidiosis.htm
Most pets that are infected with coccidia do not have diarrhea or other clinical signs. When the coccidia oocysts are found in the stool of a pet without diarrhea, they are generally considered a transient, insignificant finding. However, in puppies and debilitated adult pets, they may cause severe, watery diarrhea, dehydration, abdominal distress, and vomiting. In severe cases, death may occur.
However animals can have IBS like people do. Not really sure on treatment for those as I don't see enough of those at work to really remember what the dr's do for them.
If it was my dog I'd be inclined to go for a second opinon, fecal test, full blood work up, and possibly x-rays.
Sounds like the dog wasn't well cared for prior to your son getting it and may have some underlying issues that are still plaguing the dog.
Hope things work out ok, and the vet figures out what the problem is.
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