Thanks for the additional info.
My first step here would be running some bloodwork. I would also do a fecal examination. This would rule out kidney disease, liver disease, a liver shunt, abnormal white blood cell counts and anemia. The fecal exam will help to rule out certain worms (rounds, hooks) and coccidia. It's not going to rule out whips or Giardia.
If everything is normal, I would probably treat with a general dewormer (Panacur) and an antibiotic that also treats Giardia (Metronidazole).
If the signs didn't turn around, then I would go in surgically and explore the abdomen. I would take biopsy samples of the intestine, liver and lymph nodes. This would be to help rule out IBD (inflammatory bowel disease) or other infiltrative diseases of the intestine (Lymphangiectasia).
Here are a couple hand outs on IBD and lymphangiectasia to read over, as that may be what we're dealing with:
I hope this helps, let me know if you have any other questions.