How can I get my stuff back from my evasive, passive-aggressive ex?

Last summer I was living temporarily with a boyfriend while I took a job in NYC. It was a good arrangement for a while but toward the end of the summer, the relationship dissolved and I broke it off with him. Unfortunately when I left, I didn’t have room to take all my stuff with me. I was pretty bummed about it but he said I could come back for it whenever I wanted. I’ve recently moved back to the city for a summer internship and have been trying to get in touch with him for days about finally getting my stuff back with no response.

This is not the first time I’ve had this issue with him regarding this. Over the last 2 semesters, I tried numerous times to arrange times to pick up that stuff and he would either not respond, or if he would something would always “come up” and he couldn’t make the arranged time. I’m now super fed up and just want my stuff back! I’ve been trying for days to reach him via facebook and text for a few days and he just hasn’t answered. It’s pretty obvious that he’s avoiding me and I’m wondering what I should do. :-/ Should I get in touch with a family member? I messaged his sister on FB about it politely and she never answered me either. Should I go to the police? I don’t want to take it to that level if I don’t have to but I really want this stuff back. One of the items he has is rather expensive and another one is an irreplaceable work of art made by a good friend that is both beautiful and very dear to me.