Clavamox can definitely cause gastrointestinal upset in cats. This can lead to vomiting, anorexia, and diarrhea. I would stop the medication and contact your veterinarian. If he needs to be on medication they may want to change antibiotics. It may not be the antibiotic and there may be another underlying cause. Cats can develop secondary liver disease called fatty liver from not eating so this needs to be addressed now.