This is interesting, thanks all. It seems like some other DC do have more but his is not the lightest breakfast, by any means.
At weekends, he eats considerably more. If we go out for breakfast he will easily polish off a full-size adult Full English (or more if at a Toby Carvery, he takes a bit of everything!). But he just does not have time on weekdays & we don't have time to cook breakfasts or do smoothies.
He is not a fussy eater at all, will eat anything except about 10 foods max - current favourite foods are snails, blue cheese & asparagus 
Although he has ADHD, it is inattentive type, meaning he does not get hyperactive, rather he cannot concentrate. Used to be called ADD, I believe. He is on medication for that & we had read it could suppress his appetite, but it doesn't. It hasn't as yet had any effect on him, so his dosage is being upped.
I must admit I was rather surprised that the pediatrician was not happy with his growth. I thought that 2 cm in 2 months & 2kg in 6 months, although not huge, was at least a step in the right direction but she did not seem impressed. We went to Great Ormond Street (musculoskeletal team) in January for tests (at her request) & they said that although they would prefer his weight to be nearer the 50th centile, they were happy enough as they could see his dad (also there) was the same build. So I'm not sure which set of professionals to go with, iyswim.
I will have a think about weekday breakfasts & see if I can improve somehow, maybe even just different toppings or a smoothie instead of a juice. It's food for thought (pun intended
)