All kids and adults, have varying physiques.
Your son is 8. And is a picky eater etc.
Well, my son is 6, and is a picky eater and has always been this way since he was a baby. And my friend's sons, are the SAME way.
My son and those other boys, are lanky. But tall.
They are fine and perfectly healthy.
My son though a picky eater, eats healthy. And he is a GRAZER. And is very active. Like most boys. But he is, lanky. Grandma, would even say in front of him "he's so scrawny... I can see his ribs.... my brother was bigger than he was at that age...." etc. And boy did I get mad. I told Grandma, in front of my son, that she is NOT to talk about him that way... he is healthy, and he is fine. In fact, I took him for his check-up, with Grandma once, and told the Pediatrician that she thinks my son is too skinny and too lanky etc. WELL, the Pediatrician said, to us that my son "is PERFECT. His growth and proportions and weight and BMI, is PERFECT. And he eats well too, because he goes according to his body's cues, and eats when hungry and stops when full. THAT is how it should be." The Pediatrician, NEVER said, that my son has a problem per his physique or size or health or his eating. Because, he doesn't.
So now, Grandma has stopped nagging about my son's size.
The Pediatrician said that he does NOT need Pediasure or anything.
He is healthy.
He is fine.
He is just a lanky tall boy. And one day he will be taller/bigger than us.
I really don't fret over my son's physique.
Because, he is fine.
And although he is a "picky" eater... he eats healthy. He knows his body and eats when hungry and stops when full, and does not eat out of dysfunctional emotional eating or boredom. So he is fine.
And on his own, over the years, he does now eat more things and more varied things. And all without a battle.
Boys... do not have to be all thick and bulky and big.
MANY professional Athletes, are lanky and "thin."
And they are perfectly, fine.
Many boys, are thin and lanky.
My son's friend, is a STICK thin boy. But he is so great and SO athletic and plays football. And he is lightning fast and so well adjusted. Even if other boys are thicker than he is.