Welcome to the world of joint accounts.
No, you cannot dispute this because you are financially obligated for his payments if he does not make them. The only way to remove yourself from his auto loan is if he can get it refinanced in his name alone, which if he is late regularly is not likely.
I suggest that if he cannot afford the car, you and he sell it to pay off the note (hopefully you aren't too upside down in the note) and have him buy what he can afford ON HIS OWN.
The problem may be: He might not agree. The car may be worth much less than you and he owe. Or both.
Is he still your boyfriend? If not, then he may not care what is happening to your credit (worse yet, takes pleasure in your demise as "pay back" for the failed relationship).
I definitely think you need to have a sit down, heart to heart (if possible) and discuss what your financial and credit goals are and come up with a plan, whether that is him getting another job to afford payments, selling the car or seeking refinancing in his name.
