Grandson with huge appetite

Try and get him to do the 20-20-20. Only load your fork with 20% of the food on your plate, chew it 20 times, and wait 20 mins after you have finished your meal to see if you are still hungry. Can't say I've tried it yet, but met up with my daughter today for a pub roast and she swears by it.

A bit much for a 9 year old to do and makes a big deal of what should be dealt with as a trivial problem from his view.

Fine for adults.

I too think he may be readying himself for a growth spurt.
I can tell when my dgd is about to get taller (shes at my shoulder aged 8) because she eats like a horse and looks for more. Rarely has a tummy though. Shes all legs at the moment. But she never sits still! Even while she's sitting, some part of her is moving. She hasn't any diagnosis of adhd, just fidgety.

My grandson is 9 and has a appetite similar to your dgs.

He is growing upwards as we watch! I can hardly believe how tall he's gotten in just 6 months.

My sister, thin as a rake, used to eat constantly - a box of Weetabix every couple of days in addition to her 3-4 regular big meals and often taking a cake to bed with her.
She freely admitted she never, ever felt full.

I who was on a strict diet (no sweets, fried foods, cake etc) because I was fat from birth, constantly put on weight, eating a fraction of what everyone else ate.

There's no justice but at least now I know why it happens.

Incidentally, I have been labeled 'obese', better yet, 'morbidly obese' most of my life but it has in no way restricted me from having an extremely active life.

Even the health problems which have come with my 70's are nothing to do with being fat.
So let's not run away with the idea - much outdated - that being fat means early death and useless to society.

Kids put it on, and then take it off! Mine always did. I've got skinny photos and fat photos.

My son always got wider just before he got taller (and thinner again)! My daughter never did.

My DS had what we called 'hollow legs'.