My SO keeps eating all of my fucking food

Okay. So I have always had a hard time getting myself to eat properly. I either completely purge on food or starve myself for days. There is no in between. I've been trying to get myself onto a healthy diet and my SO (significant other) is supporting this. We bought some groceries yesterday and I bought myself 2 boxes of granola bars. I really like these ones and they help me get food in my stomach on the days I dont want to eat. I dont make a lot of money, I work part time and go to college full time. I make

Roughly $500 a paycheck so my SO and I made a deal. I will do the big grocery shop at the start of every month and pay for our internet. He pays rent and picks up little things here and there for food if we run out. Well, I woke up this morning and found an entire box of those granola bars empty, with all of the wrappers in the garbage. I am beyond pissed off here. He constantly does this. He gets up in the middle of the night and eats everything he sees. I've explained to him so many times that we just cant afford that! He has a 4 year old daughter we need to feed 2 weeks out of the month when she is with us and we just dont make enough money while I am in school. What do i do?? I've tried hiding the food but it just doesnt work as he finds it every time, I buy things as cheap as I can, I shop at the dollar stores for snack foods hoping he will eat that stuff but he always goes for my things. These granola bars are expensive as they are protein bars. I was told by my doctor I need to be eating more protein so he recommended these specific granola bars and they arent cheap but they really do help me control my erratic eating habits. My SO apologized when I confronted him about it but I know hes going to keep doing it. It's so frustrating. Hes 31 years old, I shouldnt have to tell him constantly to stop doing this