At first, I thought @ChocolateCrepe was in a shared care situation! OP, you live with the father, if you’re not happy, say so. Collate all the information first, then talk to DP!
I'm quite a bit older than most of you (although I’ve been 21 a lot in recent years) and when I was a teen, living with 5 adults and another teen (2 years older) with 3 adults being active men who worked hard, this is how meals usually panned out:
Breakfast: cereal or porridge with toast OR a serving spoon of rice with 2 serving spoons of Dahl (the men also got either a paratha or a chappati) OR egg on toast - all being dependent on the lady of house being up or not and being bothered to cook! She usually did cook and was up!
Lunch: schooldays would be either what was in the canteen or if you wanted to risk getting detention, the chippie for a small portion of chips and a coke (I was lucky, I never got caught).
Weekend lunches: usually stuffed paratha or chappati with last nights leftovers. If you had gone out, Wimpy on the high street for a cheeseburger meal! Weren’t allowed McD because their burgers were called “ham”burgers 🙄
Dinner: some meat curry, if chicken one piece each, if chops two each, or if something like potato chops (think like a scotch egg but the size of an individual pie and instead of egg, mincemeat curry wrapped in mashed potato and then covered in breadcrumbs and deep fried - my kids all still love this), usually one. Always with chappati and salad. The men got an extra piece of meat/chicken/whatever and always with one chappati each, men got two. Mincemeat curry was usually 2 serving spoons.
Sometimes we had a homemade dessert like Indian ice cream or Indian rice pudding, maybe gulab jamuns or jaleebi. Usually we all had fruit, lychee or mango, something like that.
At home, we always ate our cultural/regional food and I still do - luckily my DH likes our food!
I couldn’t tell you what the calories were that we consumed in a day, but I do know that I never felt hungry, which I think was to do with eating regularly. I think as adults we sometimes get so busy that we skip meals and then healthy eating/portions go out of the window!
I think what the DS had was fine with the sandwich. I know it was pizza night but I would prefer to see a side salad, some lettuce, tomato, quke and dressing is not hard to do. Even just use olive oil, some chopped coriander and a little lemon juice as a dressing. If the oven was on for pizza, perhaps DF could make some garlic bread with cheese?
I have to apologise as I do tend to go overboard with advice sometimes most of the time! I think it’s being a nurse! It’s always pounded into us to educate our patients, so I tend to just think everyone wants my little pearls of advice 🤨
I think you should explain to DF that yes, treats, outings, presents should be equal but food should be there on the table for people to help themselves! Even leftovers can be repurposed - side salad = salad for sandwiches! I’m going to go out on a limb, I doubt that any fries, pizza or cheesy garlic bread (if available) will be leftover. On non-pizza nights, could you just have bread and butter on the table for everyone to help themselves?
My own DD (17) has already eaten last nights leftover cheesy garlic bread, some of the pizza and is now chomping down on mashed potato from Thursday with gravy on it. Not sure what she’s gonna eat next, but there’s plenty for everyone.
I totally get that everyone in the UK is doing it tough right now so food waste would be annoying, but most food can be repurposed - I turn leftover curry into pies. Leftover potatoes can be used for so much, even little potato chops. I also always freeze leftovers!