And I'm worried

Ryan had his 1 year check up today. The peditricain said that he is healthy but he is concerned about his weight. Ryan weight 18lbs 3 oz and is well below "average" for his age. I have to take him back in one month to have his weight checked and if he hasen't gained they are going to do blood work to see if there is a reason why since he is eating more than enough food. I wanted to cry right there in the Dr's office when he said that he was worried but I kept it together. It makes me feel like I have been doing something wrong like not feeding him enough, evne though I know that is not the case. So hopefully in 1 month he will have gained some weight but in the mean time I am supposed to get him to eat as much as possible!