I have a similar situation in my house. My to eldest boys are 16 months apart but complete opposite builds. DS1 whomjust turned 8, is quite fine and average for height. DS2 on the other hand is taller than is brother, and also very solid, has a very broad back and chest and is thick. He is nearly 7. He currently weighs about 32kg. I worry as he has said a few times that he is fat (from comments at schools bout his size). I have had to explain that he isn't fat and that he is big and strong. Try to explain the difference in body shapes etc. he is very sporty, rides his bike, kicks footy, plays basketball etc. we have a focus on healthy eating, but try not to make him feel like he needs to lose weight. Sometimes I worry as he gets lodge in the tummy but invariably it is just before a growth spurt and the its gone again. I think I would remove the scales if its becoming about the numbers for your son. Maybe look at things like measurements, Height, waist etc.