My son, who is 19, 5'9" tall and weighs just 120 pounds has been tested for TB, PE and heart problems, all of which have proved negative. But he is still getting severe chest pains, night sweats, has lost 20 pounds in weight over the last 6 weeks, and is constantly exhausted. His appetite is practically non-existant. What could possibly be wrong with him? Are there any further tests I can ask for?