This is something that comes up here every once in a while. People break up, the alcoholic leaves, the other person tries to get the alcoholic/addict to take their belongings and they don't.
It's a tactic. He wants to be able to have a reason to contact you and/or see you at some vague point in the future. Being held "hostage" by someone else's crap is very annoying. I've had it happen to me.
If I had the stuff packed and ready to go, I'd pay the postage and be done with it. I'd feel I had done the respectful thing by returning his things and could walk away guilt free. Don't expect any letters you write to his family to have any kind of positive effect. It really doesn't matter what lies he has told them, they will believe what they want to believe.
Break ups are always painful and sad. There is a extra level of confusion and frustration when your ex is an addict. I am truly sorry you are having this experience.