Growth Hormone, True or not true

I have twin boys born three months premature,  My oldest son is a small built child while his brother is a little more "stocky".  My worry is that in almost three years he has only gained 1 kilogram even though he eats a very balanced diet and he eats quite a lot for a child of his frame.  He has no sweet tooth, loves cooked foods like meat and veggies.  Is not a great fruit eater, too sweet I think.  I have taken him to the Paed as I am worried that he is not growing and that this will affect his mental development more than his prematurity has already which is just a year behind considering he was 520grams at birth. Paed said if he eats I should not be worried but I am a mother and the poor child is still small and compares to a 2 year old in size but much skinnier. I heard that there is something called a growth hormone that is either underdeveloped or non existent in prem babies, is this true and what is the importance of me finding out if he has a problem or not. I have had him on Paediasure and other milk like feeds but nothing worked AT ALL.