Tell him the truth. Let him know that you're done. You see him making excuses, and you're tired of it. He's a cheater and you know that he won't change, and you deserve better. Tell him that since you know he can't leave the job, and you can't, that you'll have to work together, but that's all.
Make it very clear that the relationship is over, that you need to move on, and that if he really ever cared for you, he'd let you go and not make this difficult for you.
Then stick to your guns. Go NC as much as you possibly can considering that you have to work together, and allow yourself to get over him. Whenever you start thinking this can work, remember my SIL. She's miserable, and she wasted her whole life, and continues to waste it, on a man just like yours.