This is his stuff so he needs to come and get it. He doesn't have a place for it he may rent a storage. Like Rad 44 said give him a deadline and let him know what you're are going to do with it. For examle: "On the 10th of June I'm putting all your stuff on the curb, come and pick it up or I'm putting everything on craigslist and getting back the money you owe me". Just have a written proof ;-))).