I don’t get why you let them fill the fridge with beer. Do they drink a fridge-ful of beer every night? I presume not, so show them how to do what the vast majority of other people do which is have a stock of cans or bottles (of beer, cider, fizz, water, wine, whatever) in the cupboard, under the stairs, in the garage, and then get out what you fancy that night/the next day and put it in the fridge to chill.
It sounds like you’ve never really cooked or had meals cooked for you when younger. You can still have hot meals in hot weather! Think about Italy - pasta, meat, pizza. You just eat seasonal products with a lighter dessert, eg fish or chicken, pasta, stir fry and rice, Mediterranean veg, etc etc etc. A hot meal doesn’t have to be a steaming plate of stodge.
Kindly, your diet sounds poor, especially if you’re pregnant. Picking at things isn’t a proper meal. You’ll have to cook for your child once they’re weaned. No - not “have to” because it’s not a chore. If you don’t, they’ll grow up with unusual eating habits like you. In a way, you found your culinary match in your DP because you say that he and his son relied on takeaways before he met you, but if you’d met most other people you’d find they’d eat a proper meal too.
Do online shopping. Start to cook a few days a week and then gradually increase that. You don’t have to do all the cooking. Your DP and son could do it some nights. Get something like the Roasting Tin cookbooks because they’re easy and often have the veg cooked in with the meat/carbs.
To answer your question about your son, either cook for him or reduce the money he pays so he can buy his own food and cook for himself. I feel sorry for him with a parent and step-parent who can’t/won’t cook.