My roommate always steals my milk to drink. What should I do?

Now a sophomore, I have lived with my roommate for two years. I have never had this problem before. At the beginning of this semester, I suddenly ate my meal, that is, I bought a watermelon and ate half of it. Sometimes I just ate that biscuit. To tell the truth, I really didn't care at first, but later I was sure that I had a good relationship with my roommate. The turning point of the story was a month ago. After lunch, I ate a roast chicken and ate half of it. I want to eat it as a snack at night, and then, it's gone. Of course, roommates usually say hello when I go out to help me finish it. At noon the next day, I bought him two roast chickens and several cans of beer and told him it was my treat. He had dinner in the afternoon, but I bought him a chicken as a snack. Later, at his strong request, I didn't continue to buy. Always smiling, don't want to get into trouble with him. After living for two years, there is nothing wrong with him. That's what I told him. I'm not stingy. I don't even want to share what I eat. If you want to eat, don't help yourself. Tell me, I'll buy it for you directly. We study, eat and sleep together in class every day. Needless to say, I'll treat you for a month or two. Later, he may also be embarrassed, always taking my things by himself, knowing that I am unhappy, so it is not like this. Then we are very happy and good brothers now.