It's really not that much of a pain to change your number. It is done in business all the time. A simple email or text to all your contacts (or only those you wish) will inform them of your new number and your problem with the ex will be solved.
Other than that, I have no suggestions other than to just STOP reading his texts. Delete them as soon as you see you have one. It's going to be up to you, one way or the other, to end contact.