My 9 year old son has a similar problem. He looks 2 or 3 years younger. He is very skinny & eats a lot.
He was the same size as the other kids his age until about 4 years old. Then we noticed that he never gained weight. His pediatrician sent us to an ETM specialist. He saw right away that his tonsils were very large. He did snore. The specialist said the tonsils blocked his airway when he slept & lack of real sleep prevented the growth hormone from working. He told us to get his tonsils out.
I was afraid because he was so thin & only 35 lbs at 7 years. I feared that he would have bad effects from the being medically sedated. I waited a year & realized he really needed it to grow. He had them out & was fine!! He ate a hamburger the night of his morning surgery. The Dr. said it was fine.
He has grown slowly & did gain weight. He just had his 9th birthday & wears a size 7-8, slim. He is smaller than his friends. We are fortunate to have a great school with an anti-bullying program. He never has complained about it. That may change this summer in sports , but we will take one step at a time.
My 16 year old son was not small but short for his age until he turned 13. Some family members would say he was so short. Then he grew fast & is 5'9" now. His dad is 5'11".
Also my nephew is very short for 14. But he is known for being an great soccer player. He is a very talented bass guitar player & his band continues to win rock competitions. Also he is a year advanced in most of his classes. He has a 6' tall brother & his Dad is 5'8".
Maybe ask his pediatrician about seeing an EMT & ask about the sleep-hormone situation. I hope this was helpful. He can be a great kid no matter how big he is.