Open Question: What can i do about this credit card? Pleeeease help...?

My ex (we were never married) and i opened a credit card about 7 years ago and have since split (5 yrs ago). when we split, we made a deal that i would pay all of the rent in the apartment (even though i was not living there) and payoff his truck and he had to pay the credit card which had a balance of $1200.00. he has not paid it off STILL and the only person the debt collection agency is calling is me saying that his name is not on there, but it used to be. i have never been late on paying anything ever and this unpaid credit card is demolishing my credit score. i cannot apply for a mortgage on a house because of this ONE credit card that has dropped my credit score from 765 to 590. i even had the balance reduced down to $900 for him over the summer and he never paid so now the balance is $1800 because of fees and finance charges and i am the only one getting threatening phone calls. what can i do to get this resolved/taken care of as i cannot afford to pay for it myself and we were never married so because there were no divorce papers i feel like i have no ground. please help.