There are many other causes of unexplained weight loss like worms in stool, depression,
anorexia nervosa, AIDS (unlikely in your son’s case), cancers especially colon cancers, drug abuse, infections and loss of appetite. Loss of appetite can again be due to certain medications, drug abuse, depression, AIDS, acute and chronic infections, eating disorders, cancers and hypothyroidism. Increased metabolism is seen in hyperthyroid states, HIV, cancer etc.