Hello, I have gone through your query and understand your concern. There are numerous causes for weight loss. One of the most common cause you have to rule out is stress and Depression. Because it is difficult to identify with out any careful evaluation of possible psychological causes such as depression. Further it can not be detected by routine examination and investigations. Other possible causes are Malabsorbtion and Malignancy ( Cancer), autoimmune diseases, HIV, tuberculosis ( Abdominal Tuberculosis is common in Unexplained weight loss). Thus he need the further evaluation like Nutritional evaluation like levels of vitamins and minerals, Endoscopy, Colonoscopy, Bone marrow aspiration and HIV serology. Depending on the cause, we have to treat him accordingly. For example: if he suffering from any depression, then he need Psychological counselling and support. If the weight loss is due to nutritional deficiency, then we have to give the supplements for him. Hope this helps. Please write back for further queries. Wishing you good health.