Stacy Darling,
If he is such a wonderful guy, why would you go and sleep with your ex? You are now in a situation where no matter what you do, you'll most likely hurt the boy. If you tell him that you did sleep with your ex, he will obviously be angry/upset and you could have possibly broken all trust and made him not want to be with you. If you don't tell him, more than likely it will eventually slip out, from someone and you'll be caught in a lie, which infuriates most men. I say tell him, honesty hurts but it's always best. And maybe since he heard it from you, he'll be more forgiving. No guarantees. Best of luck.
-Jane Doe