There's a third condition that could be at play here.
If this kid has problems with constipation, he could have too much bowel. When food passes through small intestines that are abnormally long, it sucks the water right out. Vitamins, sugars, protein and fat are digested long after the stomach has emptied. End result? An "overly-efficient" digestive tract that makes it nearly impossible to lose weight.
There's only one way to fix it.
Megacolon? Or are you talking IBS or colitis?
Congenital leptin deficiency is another possibility.
Elton John's nickname?