I have two sons, 12 & 14.
Ds1 has always been easy to feed good healthy food, from when he was first weaned. DS2, despite being approached exactly the same way has always been difficult to feed.
A typical day for DS1 would go:
Breakfast - 3 egg cheese and ham omelette, large glass homemade smoothy
Lunch - in school canteen. He'll choose the meal deal, which is typically a roast, or something like chilli and rice, with a small cake and a bottle of water.
Dinner - A proper family dinner, either meat and 3 veg or something with pasta/rice.
Snacks - Cheese on toast, fruit and (too many) biscuits
DS2 OTOH will have
Breakfast - a tiny bowl of cornflakes, with milk, most of which will be left in the bowl and a few sips of the homemade smoothie
Lunch - he'll choose mostly cakes and fizzy drinks from what's on offer in the canteen. Sometimes a sausage in a bun
Dinner - he's given the same as the rest of us, but rarely eats more than half, and less than that of the meat.
Snacks - doesn't bother unless biscuits/cakes are on offer, which they aren't if he didn't eat his meal.
So, DS2 has a terrible diet, but in terms of general health, fitness, growth, energy levels, performance at school, if anything DS2 does better.