Does Melatonin Cause Fatigue And Weight Loss?

My son is 13 and has had a really difficult time getting to sleep at night. Per the advice of his internist, he began taking Melatonin at night about three months ago. In the past three months my son has lost weight and grown about 3 1/2 inches. He appears to be very tired and end up taking lots of cat naps. He has become distant and withdrawn when at home in the evenings. I know that some of these patterns are normal teenage behaviors. My question is this: could the Melatonin be causing these changes in behavior?