I know! It's preety upsetting when this happens!
Give him 15 days to pick it up. Get it out of your sight if possible.
Contact him with whatever means you have: email, instant messenger, phone, friends, family or letter.
Repeat the same message a few times to make sure he gets it:
"You have 15 days to come pick up your stuff. After that, I'll simply get rid of it. I did not hear from you and you are obviously gone forever."
It's his responsibility to take care of his things, not yours. So, you don't have to feel responsible for it. You are not a storage warehouse.
Be fare and give him a chance to get it back if he wants to. However, if he does not care about his belongings, why should you?
Will he freak out if his stuff is gone when he wants it back 6 months from now? Maybe... Who cares.
He did not care much when he took off without warning. Did he worry about you or what would happen to you next? Not at all, right?
This is a radical solution but it works and it is your right. It's your house.
You don't have to sit around with his stuff! It does not belong to you.
To your power!