I am experiencing abdominal pain, diarrhea, tender abdomen. I have had stool tests done to rule out all basic stuff, fecal fat test, blood work, ultrasound which showed cholelithiasis, enlarged spleen (14.3cm), and fatty liver. Surgeon not worried.

Many possibilities. There are many possible causes for your symptoms. The most helpful clues will come from a very detailed interview by a knowledgeable physician, preferably a gastroenterologist. Shooting first with lots of tests and skipping the questions usually wastes a lot of money and often results in chasing lots of dead ends. If careful history and directed testing are reassuring, then IBS is the most likely.
Fatty liver to start. Distinguish fatty liver (steatosis) from fatty liver with abnormal liver tests (steatohepatitis). Best to start with a weight loss diet based on calorie and fat restriction, avoid alcohol, control diabetes & cholesterol (if present). Your doctor will likely want to monitor your labwork and imaging, & consider all diseases that can promote steatosis.