Our beloved is in Y5, and is nearly as tall as Y6 boys (142cm) - however he weighs only 31Kgs [sorry I am metric only, have not learnt stones/feet/inches]
Body Mass Index of 14.88, even using scaled BMI its 16.23, its too low - should be nearer 50kg for his age.
DH calls beloved "Sidewinder" - snake and missile...both go swimming together twice a week.
We have seen dietician many, many times, and we cannot stand over him and ensure he gets 3000+ calories per day.
He is not a picky eater, except vegetables and cheese. Eats fish and chips once a week, ravioli, baked beans, sausages, ice cream, hot puddings, and hardly puts on weight.
He does not drink carbonated sugary drinks like Coke, Pepsi, Fanta - loathes flavour. Drinks mostly water and milk.
We are mystified.
Brain and mouth cannot use up so many calories alone surely.