First let me ask..... is he growing and healthy other wise? - and do you think that he eats a balanced days meal by the time he goes to bed each night?

If you answered YES - then I would suggest that you continue on with how he has been eating and just gradually add a few extra items here and there, but I would not stress to much over this or loose any sleep as some kids are just born to be thin no matter what a doctor says.

Ex: - I was a very petite child and a very petite lady when I married... weighing only 105 pounds on my wedding day and believe me I ate well, I never missed a meal or turned down a snack.