Hi and welcome to HCM. This is not liekly that diet soda would cause fatty liver. Common liver disorders is fatty liver which can be found in healthy people, but usually is resulting from elevated blood lipids, overweight and elevated blood sugar. If ultrasounds didnt find anything and viral markers are negative then you have no reason to be concerned. He should treat it by changing dietary and lifestyle habits. Do exercise, avoid alcohol, smoking, fats, fast food and carbonated drinks. Eat more fruit and vegetables, boiled meat and follow some of hepatoprotective diets. Findings usually improves on such measures. If not, then some medicines can be prescribed, but I suggest to go step by step. Wish you good health.