Loss of Appetite

Can be caused by a poor or very one-sided diet or by parasites in the gastrointestinal tract. First do a worm count.

Is the horse constipated? Are there fresh droppings? How many times a day? What consistency? Many common ailments are accompanied by refusing food.
Does he have a fever? (See Fever)
Regular doses of Biroca-Ursonne will quickly solve the problem if no other obvious cause is found: it strongly stimulates all the digestive processes or use 50ml of Samsara Digestosan every 3-4 days for a week and see what changes.