Yes. If these are his items (and your question reflects they are-- that they were left there but not by intent GIVEN to you), then he has the right to their return or a suit against you for conversion of the property. You, of course, can put the stuff in storage and tell him he must pay the storage fee to get them out. But frankly, he's an ex. He's out of your life. So why do you want his stuff? Put the feelings aside and tell him he has X period of time to get the stuff or it will go to storage (where he's responsible for the bill).