I'm going to cancel a meetup with my boyfriend and then refuse to return his things the next day
What makes you want to treat him that badly?
I'm sick of our arguing, he just does it because he can't afford to lose me.
If you want to break up with someone you just tell them. It is wrong to steal property that does not belong to you and you should return it if the relationship is ending.
It's my stuff now, he's just going to throw it after me.
I think that it will be helpful to you to just make a clean break from him. You should not keep things that do not belong to you.