My 10 Yr Old Son Is On The Low End

My 10 yr old son is on the low end of the percentiles for both height and weight. He is the smallest of his friends and is stick thin, cannot seem to put on any weight even though he eats a lot. His younger sister is in the 97th percentile and towers over him. I am 5’ 6” and husband is 6’ 2”. I am concerned because I gave my son soy formula as a baby, but not my daughter. Could this possible be the reason for his small stature? I read that it may impact either his hormone levels or thyroid. Should I take him to a specialist for tests? He is not in the lower 5th percentile, so I wondered if they would still see me and consider testing him.