You are going to have to just suck it up and tell him. You HAVE to tell him. He will find out soon enough via his own symptoms. Tell him that you understand he might want to end it, but you felt that his health is more important than your feelings. Which, it is. You have to remember that. If you are truly remorseful, and he is a forgiving man, you might get lucky, but the thought that you could have attracted a more severe, potentially fatal std, might never leave his mind. Regardless, take accountability and confess.