My name is***** and I am sorry to hear that Si is sick.
When I see a kitty with chronic diarrhea, the first thing that comes to mind is inflammatory bowel disease.
IBD can be related to a food allergy, so I like to start with a diet change to a hydrolyzed protein diet. Science diet makes Z/D, Royal Canin and Purina also make hydrolyzed protein diets, so if the flavor is not appealing to your kitty, you can try one of these other foods.
IBD can also be an immune problem where the immune system is clogging up the intestines with white blood cells leading to maldigestion, diarrhea and weight loss.
Another possible cause of diarrhea would be pancreas problems (either inflammation- pancreatitis or lack of production of digestive enzymes- exocrine pancreas insuffiency). The best way to diagnose this is with a "GI Profile" blood work (both Antech and Idexx have this profile as well as the GI Lab at Texas A&M University).
The last cause that comes to mind would be intestinal lymphoma. This can appear very similar to inflammatory bowel disease and biopsies can be needed to differentiate the two diseases.
I would ask about the "GI Profile" as this can also aid in the diagnosis of intestinal disease.
Abdominal ultrasound is also great next step for your kitty. Your veterinarian can measure the thickness of the intestines as well as look for inflammation around the pancreas.
The last step to consider would be an abdominal exploratory. This is where your veterinarian would anesthetize your cat, open up their abdomen, visually examine all of the organs and collect biopsy samples for analysis. This is the gold-standard for diagnosing IBD vs intestinal lymphoma.
I know this is a lot of information, but does it help? Do you have any more questions?