Look at your abandoned property laws.
Here, you give him notice you intend to dispose of his property and he has a certain timeframe in which he can pick them up. If he doesn't in that timeframe, you can dispose of them how you choose.
Inventory his property and take photos.
Give him official notice that his property is to be removed. Whatever you sent might be considered as that. Also let him know he can only access your property to remove his stuff and any other access will be considered trespass. You can arrange a time for him to pickup, he just can't access at anytime he pleases and he can't just rock on up.
Dear x,
Your stuff stored here on my property in xroom must be removed by x days of your receipt of this notification. Failure to pick up your stuff will make me think you're abandoning it and it will be disposed of. You only have my permission to access my property to pick up your things. Send me a note as to when you wish to do this.
Send your inventory of his stuff along with it. Here, we send these things through the mail with a signature required so there are records that someone at his address received it.
Keep your notice if you send it impersonal and business like. You are giving him notice that he gets his shit or it's out.
This is just to prevent him from claiming things are missing or damaged and a potential lawsuit. This guy likes your money and likes to mess with you.
His friends helped him with his flat, they can help him with this.
Have you changed your locks? If not, do so even if he gave you keys back.
Arrange for a friend or two to be there when he comes to get his stuff. I can't emphasize this enough! He's got it in him to abuse you and this is a dangerous time when he knows he's losing that last reasonable reason to come in your home and contact you.
I personally would hire a small moving truck and get it over to his. That might cost you some money but it cuts that cord and keeps everything safe. Still do the inventory and photos.