The doctor will most likely check the functioning of the pituitary gland of Sam. The pituitary gland is responsible for the secretion of the growth hormone. The anterior lobe of the pituitary gland secretes the growth hormone which helps a person in his growth. Sam is not growing as well as he should. This might mean that there is something wrong with the function of the pituitary gland.
Hence, the answer is 'functioning of the pituitary gland'.