Oh, ds1 is like this. Every time he grows, he becomes alarmingly skinny. He has NO fat at all, I can put my thumb and 1st finger round his upper arm, his ankles are the widest part of his lower leg, his knees the widest of his upper leg.
His pelvis not only juts out at the front, you can see two knobs at the BACK which I have never seen on any child! And his vertebrae rub on his car seat - they are regularly a bit red through rubbing.
I will put a pic of him on my profile if I can find a decent one.
Dd is chunkier - slim, but solid. SHe has broad shoulders, stronger looking limbs and a little tummy. Ds2 is in between - thin, but broader shoulders than ds1.
What we do is;
Packed lunches - slip in a Coco Pops bar for ds1, dd has an apple (which she is utterly happy with).
Tea - give him a double portion.
Supper - he has cereal with full fat milk every night - often 3 or 4 bowls. Dd has semi-skimmed milk, but they don't know the difference - just that 'this is my milk, this is yours'. And actually dd wouldn't mind, as she has the same milk as me.
Other than that, I basically don't stop him eating, because he can forget to eat, so if he is hungry I just let him. I am stricter with the other 2 about snacks, and they don't seem to mind.