I would write a nice letter and send it to his work place for his attention, I would point out that you are not going to let the debt slide, especially after the threat he made.
I would tell him that the next letter will be address to his boss, informing then that if the debt is not paid, then the matter will be passed to a debt collection agency and as the only known address is his workplace, then they will have no alternative but to visit him there.
I would also point out that if anything happens to you, your family of property, then the recorded phone call you did make will be forwarded to the police and his employers and also the newspapers who like storys like that.