Luckily I was prepared for this and made as part of the contract which pieces he was to leave. I told him that if he reneged then I would be forced to take action. He mentioned the possibility of moving out later than he was allowed and since today was the second time he threatened me, I reminded him that severe penalties accumulate for each day he stays longer as per our contract.
I'm just waiting for him to try something stupid like threatening to vandalize the furniture or his co-op. Then I could threaten to call the police since that is a criminal act, so they would be put on notice. I could also notify his employers at the High School he works at, so his job would be in jeopardy.
I hope he doesn't try further threats though. I don't enjoy hurting people. He has no respect for me and thinks he can get his way with threats. He would do better to appeal to my sense of fair play and morality.