I would send him a notice with 3 parts:
-Ask him to remove the shed
-Offer to hire someone to move it for him, and bill him back for the cost. He can contact the manager to move forward if he would like
-If he has taken no action by a certain date the shed will be demolished and/or claimed as abandoned property and repurposed elsewhere.
If he takes no action I would send one last follow up close to the deadline date via certified mail.
This stuff is a pain but it's just part of the business.