This is alwasy a difficult issue, but ultimately you have very few options to try and have him removed forom this document. Ultimately it is up to the lender you are dealing with, in order to get your your name removed they are going to have to agree with it. In most instances this will require your ex to qualify for the loan based on their own income and credit history. Unfortuneatly this is often one of the reasons you likely want your name removed and it may prove to be a challenge.
Typically lenders are very cautious when it comes to removing anyone from an account as it leaves them in a position where they are exposed to greater risk (i.e. if you can’t make payments on a joint debt there are two people they can chase for payments, but if they let your ex off, then everything will be based on you). Obviously the lender is not a fan of increased risk as they want as many avenues open to collection should their be a default. So don’t be surprised if they aren’t very cooperative.
Regardless, I suggest you contact the lender and ask them to remove your name. Depending on the current credit-worthiness of the other person that signed for the loan (your ex) they may or may not grant your request. Unfortunately, the fact that you are no longer partners does not relieve you from the obligation to repay the debt and there really isn’t any way to change this.