I wouldn't worry too much about it- my teen daughter has always been thin and underweight no matter what food I shovelled down her. I honestly think it's a case of 'some kids are thin, some aren't'.
i do remember one particular doctor, when she was about 5/6 telling her she was very thin and asking her "do you not eat all your meals young lady?" to which she, to my shame, promptly replied "I dont like mum's food"- I could have pummelled her right there and then :-(
She's now nearly 20, and has suffered no ill effects from still being extremely skinny.