A friend of mine described her DS (10) as 'putting on a bit of weight' and 'plumpish'.
When I saw him (it had been a while) I was shocked. He was fat.
But he was oblivious. Happy as Larry.
She asked me what I thought and I was honest (not in front of th elittle boy).
It was tricky. He doesn't have a diet much different to others his age, and his older brothers and sisters are all slim despite them munching on the usual teen fodder!
We worked out that he has a natural pre-disposition to putting on weight, that he's more sedentary than his peers and siblings (he loves reading and lego etc).
She tried to change his life style in the QT, but it didn't fly. He wanted that bar of chocolate everyone else was having, he didn't want to jump on the trampoline thanks for asking!
So she told him the truth.
And actually it was fine. She did it very matter of fact. She weighed him. Told him what he was supposed to weigh. Explained that is was unhealthy not to weigh that and that she was worried.
She approached it in the way you'd have to approach a medical condition. It's unfair, it's not nice, but it is what it is.