If he wasn't on the lease and you had no agreement to store his items then it looks like he abandoned his stuff. Just like if someone leaves their car in your driveway and never comes back, you have the right to have it towed. You made attempts and he refused to take action.
or tell him you are putting his stuff on the side of the road for trash pick up. His name wasn't on the lease and you had no contract to store his items.

