You might not be able to get off the  loan. The lender would have to agree to take you off, and why would they? Removing you from the loan decreases the number of people they could look to for payment; there's nothing in it for the lender to remove you.

If your now-ex-boyfriend is not paying his share and causing damage to your credit or you, you may have a lawsuit against him. You should consult a local attorney, with whom you can share all the details and documentation in confidence, to see whether you might have a claim of some kind against the ex-boyfriend.