What Causes Persistent Stomach Aches And Eczema In A Child?

My 3.5 year old boy only weighs 23lbs. His 15 month old brother weighs 21, and his 5 year old brother weighs 40. My 3 year old complains daily of stomach aches, sometimes severe. He also battles year round eczema. Despite being so skinny, he has a potbelly, but you can see every rib. His pediatrician does not want to do any testing until he is 4, however he keeps getting smaller compared to the other kids his age . For example, he is the oldest in his preschool class, but he is the smallest.