My ds (almost 8) is super scrawny.
If you saw him on the street you would think I don't feed him.
He eats like a horse (4+ sausages, huge pile of mash + huge serving of steamed veg as an example)
I worm him.
He is tall and thin. Weighing in at about 25kg. (forget how tall he is)
I often think about trying to bulk him up, but honestly, he is so fit, he never gets sick, is always running, jumping playing etc. even at almost 8 he has very defined muscles. It's just his build, it's in his genes to be slim.