I don't know the situation properly but, how old is your son? I think you've told me before so sorry I've forgotten.
Is it possible for your son to get his own food and keep it seperately? I know it's a bit harsh but it might help to have clear guidelines of what food is his, and which is yours. I mean, it's not right for the other people in the house to eat all of the food, you deserve and need the food as much as anyone! And especially as you are trying to help yourself out of this ED, you need to be able to do that and if there's never food it's harming you a lot, both mentally and phsyically.

Do you feel able to have a chat with your son and gf (or just your son) and say look, it's my money going on all of the food and I can't afford to buy this much food all the time.

An idea if your son isn't old enough to buy his own food, give him an allowance for food (what you would spend on his food anyway) and let him spend it himself on food for him and his gf and friends. And then keep your share of your own money to buy food for you.

You've done great to suggest a meal plan and to try to get people to follow it! I'm really sorry people aren't helping you with it and they're continuing to eat all the food, it's a bit disrespectful of them to take more than their share if they haven't paid for it.. sorry if that sounds harsh, but I am worried for you and you deserve to be treated properly and that means all having a realistic amount of food and not just 2 people in the house over eating and leaving nothing for the rest!