I believe you will both still be executors but he will not be able to benefit from being an executor unless there is a fee clause or unless he is named as a beneficiary elsewhere in the will
When it comes to it if your ex is not lightly to get any financial reward ie not a beneficiary it is most likely that he will renounce his appointment as executor, if not he could slow everything down dramatically and be a real pain. It depends how bitter he is and whether he wants to annoy you. At the time of obtaining the grant of probate you could apply to the court to remove him as executor but it could be argued that it was your mothers wish that he do it with you and if he is still keen to act then there may not be to much you can do about it.
The best (and cheapest) thing is to get her to change her will now !!!