Most Helpful Guy
The best thing to do is be honest with him.
Sit him down, explain the situation and apology to him if you're truly sorry, if he ends it there, you've only yourself to blame. Accept the consequences and live with it.
If he doesn't break up with you and accepts your apology. You best make sure you make it up to that boy and never do something so stupid to break his heart again.
Most Helpful Girl
Tell him. Own up to it, take the full blame and let him make the final decision as to how that will affect any potential you two previously had.