Money and relationships.

My boyfriend and I went on a trip back in October and I agreed to but the hotel on my credit card and he would just e transfer me. He said he would pay me towards the end of October. But he never did. Anyways, I didn’t really bother him about it because he is trying to pay for school. He does have 2 jobs but he spends around 700-800 on things he doesn’t need every month and still pays for school on top of it along with bills. I’m about December I asked him to get my dogs some bully sticks, they costed about 30, I told him I would e transfer him and he sent me his e transfer, when he owes me like almost 400... I’m also a student, and he makes far more then I do every month.

how do I bring this up to him?