Answer:
First - actions speak louder than words. So observe how he acts, and what he does. If he promises something, but doesn't follow through, that means he doesn't value the relationship enough to make promises a valuable thing.
Second - make sure you say loud and clear this is his LAST chance. And mean it. Don't say it's his last chance, and then back down and give him another. You need to be firm with him and yourself.
Third - give it a little time. If he breaks your trust again, leave. It sounds really really really hard, and it is. But in the long run, it will be so much more fulfilling for both of you. He will learn that you're not kidding, that he needs to be truthful. And you will be able to find someone who values you the way you should be valued.
Have faith, and pray for guidance! God bless you.