Mean Boyfriend??

Brittany
Okay so my boyfriend and I live with his mother, sister, and uncle, in her apartment. My boyfriend just graduated from college and now he's back home full time. The situation is that my boyfriend found some Lucky Charms cereal cup bowls on the counter in the kitchen, he didn't know whose they were and he ate them. This morning his mother asks him about it and she's kinda mad. Him and I usually buy our own stuff (food &etc) and keep it in our room. I told him he was wrong for eating her cereal and he just doesn't get it. When she asked him about them he said "yeah, and they were good to". He used the excuse that they were in the kitchen and last time he checked she bought things for the house. Also we just found out that his mother started to hide the toilet paper because it was being used up. While I agree that was wrong and childish on her part because we are all adults in her household and we can effectively and efficiently communicate. I feel like my boyfriend is downright wrong for eating somebody else's food and not asking or anything, he says it's just Lucky Charms, it's not a big deal. It is a big deal because you can't eat other people's stuff that you know you didn't pay for, especially if you keep your stuff separate. I told him that he lacks compassion and that two wrongs don't make a right. 
What do you guys think? 🤔