Not rtft but have read your posts op.
He does sound really fussy but that's not uncommon at this age, I had same with dd. On advice from a fab hv I involved dd in shopping, preparing and eating. Sounds weird but it works 'ooh x new thing is on offer shall we try it?' 'want to help mummy cook?' even at this age they can get stuff out fridge, rinse at sink, help decide how much to cook, what to have with x tonight (some odd combinations like sausages and cous cous but it's still protein and carbs), also letting them 'steal' stuff off your plate.
Also until the end of the thread you were often commenting 'i don't like x y z' I must admit that annoyed me as my mother did that, the other 3 of us could love a thing ( coleslaw was one I remember) but if she didn't like it, she did the food shop and she just refused to buy it!
Like maybe he likes subway partly because he can CHOOSE what to have on the sandwich quite precisely.
As for how much, yes childhood obesity is a problem but if they're eating healthy foods there's no reason for that to happen.
This is an idea what my dd ate at 5 (once at school and last the fussy pita bit)
Breakfast at home - 3 weetabix full fat milk and a banana and a glass of juice.
Breakfast at school - toast and full fat butter, f fat yoghurt, apple, glass of f fat milk
Snack - cereal bar and apple
Lunch - school dinner, usually meat and 2 veg plus gravy type meal, pudding usually sponge and custard
Snack at home 2 crumpets and butter, glass of milk 2 plain biscuits
Dinner pasta and sauce or chilli or store fry
Dessert ice cream with fruit usually banana, or rice pudding
Supper French toast with cinnamon milky hot choc (not much choc just enough to turn milk brown)
And she is not and has never been anything but ridiculously skinny! She's currently aged 15 5'7" and a size 6-8!
Some people just have a high metabolism, she takes after my dad's side they're all skinny as and eat CONSTANTLY