You will need to get a court order, or include it in your disolution that "he has xx many days to remove his belongings from the residence and after that date you are free to dispose of the items as you deem fit"
If he is still not moved out of the due date dispose of his belongings.

It's amazing how fast they will come if they see a dumpster in your yard. My ex refused to get his belongings as well, He was to remove them before I moved back into the home, they were all there when I got to the house so I started the calls, from August to December, then got tired of it and rented a dumpster.