I agree with all above me. Just tell him how you feel, and be done with it. Above all, don't try to reason with him because as krhea75 so aptly put it, you might as well try to reason with a toddler. Don't expect him to act like a rational person, because he is not.
You obviously have been a little way down this road before, so you know what is coming. The pleas and promises, the lies and denial, the manipulation and blaming.
Don't get into a 'discussion' about any of it, because you will likely end up frustrated and dizzy. You have a perfect right to decide how you want to live your life and what you will allow in it.
Block his phone number, his email, whatever you have to do. Then go have some fun with your kids!