In that seated position, he could be hiding a really, really large belly, and with the loose shorts, you’re not seeing the size of his thighs or rear.
I have a son who is over 6’1” and weighs around 240. When he’s standing, you can see that he’s overweight, and if he’s in his bathing suit, well ...
My son is almost 16. For a boy so tall and heavy at age 12, an endocrine workup might be a good idea.
True. Maybe that is why they took the picture like that - to hide his belly.