Well, my kids all weighed themselves last night.
Ds1 who is nearly 9, and average height, weighs 4 stone 1. He is slightly built, and thin.
Dd who will be 7 in August, very tall for her age, and broad shoulders, but slim, is also 4 stone 1. She is nearly the same height as ds1, but a more solid build, but very fit and slim.
Both these 2 are older than the boy in the article, who is reception age. Ds1 is Y4, dd is Y2.
Ds2 who will start reception in September, and is nearly 4, so the most similar in age to the boy in question is 2 stone 6 - ie over a stone and a half lighter. He is small, but doesn't seem overly 'thin'.
So from my kids' weights, I reckon the boy is quite a heavy boy - I was surprised how much the difference between ds2 and this boy was, and also that dd (who I think of as being quite solid) weighed less, despite being nearly 2 years older. She is also very tall, so it isn't a height thing.